I don't see anything wrong with getting your initial breeding stock at a pet store. If you buy the "small" size of rats, they will probably be young ones, which will be ready breed after they grow up for a couple more weeks. Since starting my own rat breeding gig, I have read up on it more and learned that female rats who don't start breeding before they are eight months old will have their pelvic canal fused in a narrow position and may have trouble getting the babies out. It happened to one large female rat I bought at the pet store, and she died trying to give birth. Poor thing! So, it probably is best to start with younger ones. Once you have females that are ready, they sure can reproduce quickly. They go into heat every 4-5 days and almost always have litters of over a dozen. It really is well worth it to breed your own rats if you have a good-sized collection of snakes. Good luck!