Wal-mart, for one. Check in the athletic clothing section. Or you can get better ones from most sporting good stores that carry clothing. I have two Nike shirts I wear, and some Wal-mart ones I don't like as much.

Weightlifters wear them. My riding buddy Dennis, a pumped-up Cyldesdale, suggested I try them. He's wearing one under his tank in the photo on my blog:

http://historian2wheels.blogspot.com...y-10-2007.html