my advice is to try the phone book and call several school districts. Many schools sell their buses at the end of the school year. My local bus garage doesn't sell any until the new school year starts, it's one of those things that varies from one district to another.
I like the idea of purchasing from a school district rather than from a dealer or a person. Schools perform all of the required maintance at regular intervals, and all the records are available for you to look at. Plus this can be a really inexpensive way to purchase a bus. A year and a half ago i purchased a 1991 Ford/Ward full size 72 passenger bus with a diesel engine and auto trans for $775.00. I've driven over 10k miles so far and the only mechanical issue i had to resolve was a water pump.
Good luck !
Quis custodiet ipsos custodes (who will watch the watchmen?)