Are they for a real special application? Any reason you can't run the Napa brand ones? They are highly rated pieces and should be easy to get ahold of.
GM dealer would be another place to check, or look in the phone book and see if anyone local is a Delco distributor.
If you can get the GM part number, (not delco) you can probably order it through gmpartsdirect.com. If you can't find it, email them and they should be able to cross that with the GM part number and give you both the GM part # and cost.