LennyJoe,
If you can't find one when you get there, look in the yellow pages. There are a few listed there, and one is located at or near the Medina Annex. If you still can't find one, make a call to the Lackland Combat Arms facility, there's bound to be a couple of shooters out there.
Also there is a gunshop right off base (can't remember where though) that is owned by a retired Combat Arms Instructor. He would probably know.