Welcome to the group! You'll find a wealth of material here.
As a bit of friendly advice (and I mean that - I want you to have a good time here), try to search for what you're looking for before posting. Most questions have been answered before, and at length, by thoughtful, knowledgeable people, and you get your results immediately.
But to answer your question, there are subforums for
members selling a bus and for
non-affiliated buses we've found. Off-site, the favorite places discussed are
GovDeals.com and
PublicSurplus.com. And, of course, there's Craigslist and eBay.
Again, welcome!