best bet is a Junkyard or ebay has them quite often
here is a set....http://cgi.ebay.com/ebaymotors/1981-Buick-Regal-Cutlass-aluminum-rear-brake-drums_W0QQitemZ4628692565QQihZ002QQcategoryZ34199QQssPageNameZWD1VQQrdZ1QQcmdZViewItem
even though this person price is absolutly moronic. You can find a desent set for a good price from time to time
The one real problem is that if you dont see them you really cant tell how many times they have been cut or if they are all scored up.