EDIT: Apparently my reply is too long, so my sources are in the first comment.

EDIT2: Typos.

Hey there, I'll give it a go.There has been a lot of talk among historians (and many more) about what sources Machiavelli used. Perhaps his use of examples from antiquity has also struck you as odd, it has certainly caught the attention of many historians and it's been the topic of a lot of debate, none of which seems to ever have gained much popularity and I'd carefully posit that this is because all the writing on the subject hasn't turned out much insight. Perhaps the historical context of the time of his writing (fresh out of prison, lost his job and livelyhood, had to leave the city, was never acquitted, was tortured) contributes to this. but other than the fact that writing The Prince was clearly also something Machiavelli did to cope, this is mostly just speculation. A lot of Machiavelli's correspondence is still extant, the most famous of these is probably his letter to Francesco Vettori dated to the 10th of December in 1512, there are translations of his letters available in English.* In it you'll find a lot of what you'll also find in the dedication of the prince, namely that it is the result of his careful study of antiquity, some insights into the writing process, but what makes it famous is probably the unusually peotic tone when he describes his simple life in the countryside. If you do ever read a collection of his letters, pay close attention to his other correspondance with Vettori - Machiavelli clearly uses his contacts in high places (Vettori at this point being a legate of Florence in Rome) to get his hands on whatever manuscript he can find and his study of antique manuscripts does indeed play no small part in his life.

Machiavelli lived during the period now coined as the Italian Renaissance. While the characterization of this period as one where "antiquity was reborn" has rightfully been called into question, this is certainly a time of renewed interests in the authors of old and Machiavelli was among the most studious of readers of antique texts and it is often very hard to tell when he is quoting, paraphrasing or simply inspired by this or that author because he is very liberal in doing so, sometimes mentions where he is getting some information from, and sometimes doesn't or just assumes his readers will know.

As to the specific influences and sources - these are as you have rightly assumed Xenophon** to a very large extent, Aristotle and Herodotus to a lesser. Machiavelli never learned Greek and only could use what was available to him in translation into Latin. Beyond that a major influence was Livius, he also played a certain role in his Childhood, and I'd like to use this oppurtunity to ploy one of the best academic biographies I've ever read, wich is the biography on Machiavelli written by Alexander Lee***. The biography is very important in interpreting The Prince to me, because more often than not I find that Machiavellis actual opinions stem from his experiences and frustrations in the service of the Republic of Florence from 1498-1512, and that he finds some fitting author from antiquity to demonstrate his point after the fact. The short answer to the question of Machiavelli's use of Livius is that Machiavellis general idea of Roman history is clearly the picture that Livius paints. Then there is Tacitus, which is at least responsible for one of the few direct quotes in Latin within the Prince (which was otherwise written in Italian, or Tuscan as Machiavelli insisted on calling it), Kenneth Schellhase wrote about it**** and I was pleasantly surprised that he explicitly discusses what manuscripts might have been available to Machiavelli. The Material Machiavellli uses predates the first printed edition, so his use of Tacitus indicates some active seeking-out on his end. I guess as a final pointer there's Marcia Colish who was written about Machiavellis use of Cicero*****. The list could go on and on but these are some examples that I found useful for my own studies.

I'd also like to point out again that perhaps you will find the results somewhat vague and unsatisfactory. To my knowledge there is no "complete summary" of Machiavellis sources, and although writing on Machiavelli seems to be quite popular in academia again at the moment, I have my doubts about this ever happening. There is however an annotated bibliography by Ruffo Fiore in Italian well over a thousand pages long from the 90ies in case you have a masochistic streak and want to doublecheck. I think you might also find, if you pursue this road, that many authors are quite "over it" in the sense that they will frequently remark how overdone this topic is, all while actually having quite the difficult time trying to find these resources. As is so often the case when researching literature though, eventually I did end up with what felt like a mountain of literature about Machiavelli's sources and it was broadly discussed in Germany, Italy and English speaking countries all throughout the 20th century but seems to have fizzled down to a slow burn by the 90ies. There are some newer contributions to the topic, one might be Quentin Skinner "Machiavelli and the Misunderstanding of Princely Virtù" in "Machiavelli on Liberty and Conflict. Edited by: David Johnston, Nadia Urbinati and Camila Vergara, Chicago 2017, or Skinner again in his 2018 Book "From Humanism to Hobbes". Both are more concerned with Machiavellis more general relationship to antiquity, though, and I'd suggest that is the case with most post-2000 literature about Machiavelli, when and if they do discuss his relationship to antiquity.

There is a of course a lot more: completely out of the picture are Machiavellis more contemporary (or nearly contemporary) resources, like Dante and Petrarch. As far as I can tell he downright revered Dante, even calling him out from the realm of the dead in one of his later works to explain to him that the language he uses is not, as Dante called it, "Italian" but is indeed "Tuscan"******, and these people were themselves steeped in literature from antiquity. All in all it gets rather complicated trying to decipher Machiavellis sources and influences from antiquity pretty quickly, as an example I'd point to Machiavelli's use of the metaphor of the lion and the fox which is used by both Dante in his Inferno and Machiavelli (Prince XVIII, 4, verse numbers from the critical edition by Giorgio Inglese) and seems to include both references to, Cicero who seems to be the more immediate source for this metaphor (which in itself though can't be credited to Cicero as the inventor of the metaphor, though) but also seems to allude to the specific way that Dante had employed the metaphor.

Back to the question of Machiavelli's sources, though: He has clearly read whatever he could get his hands on and enjoys quoting from it, also (with regards to the aforementioned letter), writing is not so much something he did for the purpose of producing literature, it was an end in and of itself. He frequently claims to have gained his knowledge from the study of antiquity, but there are political reasons why he can't equally stress the knowledge he gained as a diplomat in the service of the Republic of Florence, which is a whole different topic (in short though Machiavelli wrote the Prince to curry favor with the new Medici rulers in Florence who had just recently reentered Florence and stressing one's importance in the old regime would have in no way been a smart move).I could go on - I spent some months this year researching exactly this topic for my Bachelor's Thesis (results pending, so it may yet turn out that I'm a dumbass and nobody should take my advice ;) ), but I think you've now got enough resources to join the confusing, broad and honestly sometimes very tiring discussions about Machiavelli's sources.Finally, did I mention the Biography by Alexander Lee is really great? One of the best things about it are the extensive footnotes. I haven't got the book anymore but there's a whole page long footnote on Machiavelis sources from antiquity, I think you'll find a lot of the sources I recommended in there as well, and many more. I guess as a final pointer I'd like to suggest Josephine Jungić's biography on Giuliano di Medici*******, who was the originally intended recipient of The Prince. It's got some very bold claims but has been favourably received and reviewed by Lee (mentioned here) and Robert Black, another reputable scholar of Machiavelli and the Republic of Florence during the Italian Renaissance. Both of these aren't concerned with Machiavelli's sources in The Prince, but in my own study of The Prince and trying to understand why he chose to write this or that I have found it very insightful.