any place that does head unit installs will have them.....the GM antenna adapter is one of THE most common parts in the car stereo industry...
best buy, circuit city, ultimate electronics, car toys, hifi buys, tweeter, etc.. will all have them...
Some will have them on display....if not, just find one of the guys that works in mobile audio, and simply tell them you need a GM antenna adapter to hook up an aftermarket radio, and they'll know exactly what to get you....
Its around $10-15 depending where you go and what brand you get...