Yes to Hotrod's comment about Classic Muscle through the Modern group. I've been in their warehouse several times and it is well stocked. I've found them easy to work with and knowledgeable. Of course, there will always be someone who has had a bad experience if the place does much volume. Modern's body shop is doing some work for me on one of my classic cars right now. They actually have a GM restorer on their staff.
And Buddy is certainly right about NOS, but you often have to be patient, ask good questions (to the dealer after you've asked on CRG) and hunt to find it. Sadly, there are very few junk yards with original First Gen parts left.
And of course, yes to what Cory is offering. If you can get GM at wholesale, what a bargain!
Obviously price and shipping costs are important, but in the end most of us are looking for correct and functional.