Radio shack, Walmart, Target, any discount department type store really. Although you're bound to mostly find the small portable battery powered ones. If you want a decent 12VDC one then you're going to have more trouble.
Dave of Dave and Julie fame had one that he used to install a Sirius receiver in his company leased Audi. He seemed pretty happy w/ it. I can't remember the maker or where he bought it though..