Why do you want a metal thumb pick? I have one that I never use (no brand unfortunately) because to me, it makes a really nasty grating soung as it slides on the wound strings.
Some music stores here in the Phoenix area carry them, but I guess you don't live near a large metropolitan area.
If you want to find it in the store to try it first (so as not to put your resources at risk, or to avoid paying $4.00 postage on a $.98 purchase), find a store that sells banjos, and they have a good chance of carrying metal finger picks.