Once in blue moon a bay window bus will show up at Pull-A-Part (
http://www.pullapart.com ) and every now and then, one of them will actually have a good windshield. I think they are $28. I got one last year from the Atlanta South yard. You have to be REALLY careful if you install it yourself. It's do-able, but it doesn't take a lot of presure to crack 'em.
I can't think of the name of the place, but I think the shop that's next door to the VW dealership on International Drive has installed windshields in some other club member's bay windows at a pretty reasonable price.
I think Pull-A-Part knocks a hole in the gas tank to drain them, so I wouldn't go there shopping for a used gas tank. I've never seen plastic replacement tanks for buses, but then, I've never really looked for one. I would think that most of the mail-order places would have a metal one. Check
www.CIP1.com or
www.mamotorworks.com