Most albino reptiles in captivity are descended from either wildcaught parents that were bred together and produced albinos, or from wildcaught albinos. The chances of finding a wild albino are extremely slim, as are the chances of being lucky enough to find and breed a wild pair of animals that carry the gene. Albinism seems be a recessive trait, so if one breeds an albino to a normal (thats not carrying the gene) you'll get all normal offspring that are heterzygous for albino. Breeding those offspring back to each other, statistically you should get 25% albinos.
Some albino snakes were created by crossing an albino of one species to a species that didnt have a known albino in captivity. This is generally frowned upon by a lot of herp breeders.