I contacted several dealers last year, and new will set you back anywhere from 130 to 180 bucks per side. This, again, its NOS GM.
Napa only stocks iron replacements (same with all the major chains).
Good luck finding the part numbers. They've changed them a couple of times - Ithe part number on GNTTYPE.ORG has now been replaced I believe.....anyway, give the number to a dealer and they'll find the cross ref to it.
For what it's worth, I found a used pair on ebay with plenty of material left - the guy bead blasted them and everything. If you drive limited mileage, you probably won't ever wear them out. But then again, I don't put that many miles on my car....
HTH
Rich