Primary sources from this event seem to be rare in quantity and there is wild debate over which ones are actually from the conquest and which ones are not.
I would start with this section of the subreddit booklist. /u/drylaw knows more about this topic, but please be more specific about your area of interest. I assume you are looking for written texts, but there are other types of primary sources available. If this is a homework question, I imagine your professor has already assigned you Visión de los vencidos/The Broken Spears.
/u/400-rabbits has previously answered Why was there so little written about the Aztecs between the 17th and 19th centuries? and offers suggestions to counter the OP's assertion.
/u/anthropology_nerd has also written about why the "Spanish Conquest of Mexico" is not a linear process and took a long time: