Baritones that "play" most often go for at least $500 on ebay. Things cheaper than that will be in horrid shape and/or will be missing keys or a neck. No one could tell you where to find what you are looking for because baritones don't sell for that little in an established marketplace. You would have to scour pawnshops, fleamarkets, estate sales, and public auctions, and the possibility of a baritone even showing up in one of those places is remote. The most likely scenario is a school getting rid of an old instrument, and in that case it may not be playable. The only thing you could do is basically look and wait. If I knew where to get a playable baritone for $200, I'd purchase it myself.