Your local and neighboring school districts, if they have their own bus barn, will get you probably the best. They'll have the service records, the mechanics will know what it was like in service (was it a maintenance pig, reliable/unreliable, etc.). Govdeals.com, PublicSurplus.com will usually get you the best buses on the interwebz, especially as we move back towards spring, and buses being lifecycled from school districts. CL and FleaBay are useful, but a lot of what is on there may need a large grain of salt.
And, as always, YMMV