I'm doing a project and I'm at my wit's end, my professor has been no help in this regard.
I'm looking for primary sources regarding Saint George, Saint Theodor, Saint Michael, Saint Margaret of Antioch, and Saint Marcellius.
For context I'm doing a project about historical depictions of dragons in medieval Europe with special attention being paid to bestiaries and saint vitae/hagiography
Does anyone have any ideas about where to start looking?
Any insight would be much appreciated.
The most standard collection of medieval saints' lives is Golden Legend (Legenda aurea) (linked to the selected English translation by Penguin), authored by Jacobus de Voragine in the late 13th century.
This site is an index of Golden Legend (with the links to the section available online), and it at least has a link to the modern English translation of the life of St. George in the collection, apparently based on the 15th century Middle English translation by William Caxton. Developed version of the lives of St. Margaret [of Antioch] seems also to be found in this collection.
On the other hand, the Cult of Saints in Late Antiquity (CSLA) database offers the basic (searchable) information of the cult of saints in Late Antiquity and its related evidence.
I suppose you can find an entry of Saint Marcellius there, though not the English translation of the hagiography itself.
As for the dragon-killing episode of Theodore and George, your teacher might expect you to identify the relevant text by checking the note section of the introductory article as following, I assume:
The following article has an English translation of the early miracle of St. Michael at Mont St.-Michel (9th century), but the dragon is not featured in this famous text: Arnold, John C. "The Revelatio Ecclesiae de Sancti Michaelis and the Mediterranean Origins of Mont St.-Michel." Heroic Age 10 (2007).
French-Latin bilingual online edition of the Revelatio ecclesiae sancti Michaelis archangeli in Monte Tumba is also to be found in: https://www.unicaen.fr/services/puc/sources/chroniqueslatines/presentationAcc
In addition to u/y_sengaku's excellent answer, I might mention the Penguin Book of Dragons, published in 2021. (You can get some idea of the contents by looking at the TOC via Google Books.)
On the one hand, this is a collection pretty much entirely curated towards your interests, though it runs much broader genre-wise than just hagiography; on the other, you have to be careful not to rely too much on such curated collections, since there may be relevant material for you that's not included there for some reason or another. So I'd suggest using this as an auxiliary source to introduce you to texts you might not find in standard databases (e.g., the Byzantine texts), but not as a replacement for the corpora mentioned in the other answer.