Other than in a few states where they are endangered, the black rat snake is extremely common and readily available. If you look on the colubrid classifieds of KS you'll find normal black rat snakes, albino, leucistic, brindle, white-sided and even combinations of those traits.
One piece of advice. Check on your state's wildlife laws. There are a few places like Massachusetts where black rats snakes are restricted both in terms of capture and even ownership and the law enforcement guys take this stuff very seriously and from what I've heard can make it very unpleasant for violators. But on the upside there are plenty of very close relatives of the black rat snake (yellow, everglades, grey, texas and even some intergrades) which will probably be just as good pets if the black rat isn't allowed (and vice-versa) and they are just as cool to keep. I've got some orange phase texas rats and some fox snakes and some asian rat snakes and they're all fun in their own way.
Sorry for the long winded speech.
Regards
John