Once in awhile I'll see these items on eBay. Currently there is a seller selling various parts for Roadmasters, Caprices, Impala SS'.In their auction listing, they say they are currently dismantling many Roadmasters, so if you are looking for a specific part, just email them and ask if they might have it. Here is a link to one of their auctions.
http://cgi.ebay.com/ebaymotors/1993-...QQcmdZViewItem
Click on the link in the auction listing, "Ask seller a question" on the right hand side of the page. You can then email them to see if they have any power door lock and unlock buttons.