Both big school busses, and short busses, along with lots of cargo vans and things like ambulances can be found for good prices at publicsurplus.com
I like this better then the GSA auction page, that stuff seems to go for full retail.
We bought a nice 3 year old highway patrol car off publicsurplus. I saw a running, driving 5.9 cummins bus go for under $1000 in my town. The motor and it's Allison transmission was worth 3x that and the bus would scrap for $500.
Here is an example, this will go cheap because it needs a rebuild but the dual rear wheels are better against rollovers then single, a wheelchair lift can be sold for what the bid is at.
http://www.publicsurplus.com/sms/auc...ew?auc=1660563