Q. Where do I find primary sources before the 1500's?

I think this is going to have to be a two step process. You will probably need to read some articles about gluttony and see if they mention any pre-1500 authors who comment on this sin. From there, using the bibliography, you can track down the original, or primary source. Here as some options for that hunt.

a). We have a whole print book in the Library devoted to the sin of gluttony.

b). I bought this book 16 years ago just to answer this question for you (!). Encyclopedia of Comparative Iconography: Themes Depicted in Works of Art. Gluttony appears in different articles. Use the index on p. 1086 for synonyms. Both the articles on drunkenness and excess discussed what the ancients thought.

c).  On our encyclopedia page, use Reference Universe to search on gluttony. This will pull various works like the New Catholic Encyclopedia, The Dante Encyclopedia, The Encyclopedia of Clowns and Tricksters that will give you various aspects on gluttony. These will be brief articles, so remember to check their bibliographies to expand your sources.

d).  You could also take a peak at these articles in ATLA. Some discuss pre-1500 authors.

Good luck in your search.