Iknow there is a post somewhere here but finding it is always tough on the forum...
som,e the big pnes that get alot of school busses..
http://www.publicsurplus.com
http://www.govdeals.com
# texas busses when they have them
http://www.onlinepros.com
# sometimes some good busses here
http://www.purplewave.com
422 Sales
there are others but those are the big ones that come to mind.
often in the auction listing the school system name will be noted so you can contact them directly about a bus that is listed.
thingto remember is that school busses depcriate rather quickly... the fact a bus may have cost 85k in 2003 doesmt mean its worth 10k now!.. lots of good busses can be had in the 3-5k range or even less depending on what your requirements are and how many busses are for sale in various venues..
-Christopher