These are pricey, but they're probably the best upgrade, comfort and safety-wise: http://www.gowesty.com/ec_view_details.php?id=2738

Bus Depot has some a little cheaper: http://www.busdepot.com/details.jsp?partnumber=111707BK

I don't know that I would install cheaper used belts. If the vehicle they were pulled from was ever in a crash, the webbing would be stretched and they really are really designed to work once.

I've been eyeballin' the seatbelts in my 84 Chevy S-10 . . . I think they would be pretty easy to mount in a bus.  They are retractable and work like a charm . . . and they should be plentiful and cheap in a junk yard.

For example, Pull-A-Part in Atlanta (South) has about 20 S-10s on the lot right now.  This doesn't count the S-10 Blazers, GMC S-15 Pick Ups and S-15 Jimmies that would have the same belts.  All of their seat belts are $6.89 per person.  Next time I'm down that way, I'll grab a few sets to play around with.  

Here's another option for new retractables, JC Whitney sells some for about 80 bucks: http://www.jcwhitney.com/autoparts/Search?catalogId=10101&storeId=10101&sku=12FA6244U&x=13&y=12