If there is any kind of Japanese cultural association near you, you could call them and ask if they know a shamisen teacher, who should know where to find 'em.
Lark in the Morning, a place in San Francisco, sells Asian instruments, but I've been told the quality isn't high. I don't know whether that's the case, but even if it is, I suppose it might be OK for a starter instrument? I have no idea. Even if there's not a shamisen on the website, you might contact them.