I don't consider myself anywhere near an "expert" but I do raise rats and mice. I haven't had to purchase a rodent for a long time now so I must be doing something right. I just went down to the local pet store one day and bought a male and two females. I started out with white rats and along the way I noticed a patterned rat and held that one back. I always try to hold back breeders that are friendly or have an interesting pattern.

The two things I would point out...

It's a ton of work, and it's all about the scheduling. You don't want to wind-up with a bunch of rats that are too large for your snakes to eat.