which keyless remote?

Would anyone know how or where I could find which keyless remote to order?? I purchased the van used with no remotes but have had the dealer verify that it does have it installed. The four dealers I called were very vague and did not seem to want to reveal the part numbers, just that they would order the one for this vehicle. I do have the program instructions (thanks to you all) but need to figure which remote to get.
Any help would be great. Thank You

I have also contacted GM who just referred me back to the dealer. :disappoin

Check this:
www.keylessride.com