You can usually get a rosy boa from a reputable breeder for the same price or less than you can get one from a pet store. And you'll get more info on the snakes background, locality info, etc.

There are breeders all over that will ship rosys fairly cheaply. If I were on the east coast, I'd buy from John Columbus.
He's reputable and produces really nice rosys. I've bought from him before and have been very happy with his animals.
Good luck!

(I hope this isn't seen as an advertisement. I'm just trying to answer the question.)