Here's a reply I wrote in another thread:
Okay, I think I found the part numbers. Just take these numbers to gmpartsdirect.com and plug them in, put them all in your shopping cart, and you will get a price with shipping. Then you can go to your dealer and see if they will come close to the price. I prefer the dealer over gmpartsdirect.com b/c you can't return anything (so you better have the right part #!) and all communication is via email. Here ya go!
#25787558- Rail
#25787557- Rail
#25787589- End Cap
#25787590- End Cap
#25787559- End Cap
#25787560- End Cap
2 qty #15916756- "Plates"
I got my roof rack for my Yukon XL (XL slightly more $) for $370, then I had to get the end caps painted.