any junkyard with a school bus....i looked at one in my local scrap yard and posted a thread here thinking about converting it to a waste tank.... but ended up going with poly waste tanks. i think they wanted $50 for the tank and mounting cage.
i also see a lot of fuel tanks at equipment auctions.
good luck
here is my thread with the tank i was looking at...
http://www.skoolie.net/forums/f13/ga...tank-8606.html