I got my first BP (Delilah) at a pet shop, but this shop happens to be a well-respected "veteran" in the business - East Bay Vivarium in Berkeley CA. She was perfectly healthy, mite-free, and is still doing great today!
The snakes I've gotten since have come from breeders, expos, people on forums, and I also got one more snake at East Bay Viv... my beautiful little Jungle Carpet Python named Kipling.