Try the Chatfield Brass Band Library in Chatfield Minnesota. They have the world's largest collection of Concert Band music and will have lots of things you can use. Borrowing from them is easy and inexpensive.

Look them up in Google; they have a web site with a catalog - it may take a few tries to get the right results, because it's a little quirky to use. You need to know the specific title, etc. before you order - they don't have staff to look up requests like "please send me some folk music."

I used to play in a "traditional" concert band and we found missing parts for 100-year-old pieces as well as full sets of parts for recent arrangements.