As stated...do your homework first and decide what will meet your needs. Size, type, headroom, engine, tranny, manufacturer, age, condition. Then talk to some folks who have lived with your top picks for a while. Once you decide what you want, then consider the many different ways to buy it. Craig's list, auctions, resellers, direct from school districts. There are pluses and minuses for each which have been discussed here.
One example...buying direct from a school district you can usually get a bus for the same price resellers pay and then mark up...very often you can get your hands on the unit's maintenance logs (worth their weight in gold)...and occasionally, you can actually talk to the tech's who maintained it.
Not really much on the downside going that route except for location and finding a district that has the unit you are shopping for.
The other methods have all the usual risks associated with any purchase of a used item. Most of which involve either mendacity or ignorance regarding facts about what the seller knows. ("It ran when it was parked".)
On the upside...resellers have the biggest inventory.
I wound up dealing direct with an owner but only because what I was looking for was relatively rare.
Best of luck on the hunt.