Not many reproduction parts are made for the g-body (78-87 Regal/Cutlass/Monte/Elco/Malibu). What parts are you looking for? Your best resources are salvage yards. You could also try...
www.gbodyparts.com
I've done business with Brian Weaver (at above site) and it's legit.
Also if you've got the cash you can order parts from GM, or get GM part numbers and go to GMPartsDirect.com (watch out for that shipping charge...)
Post back with the parts you're looking for and I'll try to give more helpful suggestions.