Some years ago when I was looking for obsolete parts for my Monte the Parts guy at the Chevy dealership gave me a name of a place but I don't remember if it's one of the ones I've listed below. You might go by your local dealer and check with them. I came up with these by doing a search for "GM Obsolete Parts".
http://www.gmobsolete.com/
http://www.friesenchevy.com/obsolete...nued_parts.htm
http://www.oldparts.com/GM/gm.html
http://www.obschevy.com/
http://www.gmnosparts.us/index.html