Generally speaking if you want the best deal you buy from an auction site. A real one though, not ebay. Govdeals and publicsurplus are two of the big names. The caveat with buying from these auctions is that they're nation wide marketplaces and the bus you want may be on the other side of the country.
If you're not willing to travel or don't want to deal with auctions, you can pay more for a bus from craigslist. They're not high volume type items where you'll see a bunch moving but you can also find them sometimes on facebook marketplace or apps like letgo.
If you don't want to do that much effort you can pay even more for a bus from a dealer. Dealers buy buses from auctions and drive them home, sometimes wash them, and then sell them for tripe or more what they paid. This isn't always a bad deal, consider that getting one of those sweet PNW buses in Florida will cost you a substantial amount in fuel. And a bus from a dealer was able to make the drive from wherever it was bought to wherever it is being sold.
I've bought 2 buses. One from craigslist and one from facebook marketplace.
As far as tall buses go, you're more likely to find high headroom on later model (2001-2003) with wheelchair lifts, because of various funding programs at the time.