Dont buy adults, you will end up disappointed. Rats have a fairly short "shelf life" as viable breeders, so purchase them at around the medium size.That way it wont be too long before they're producing for you, and ensures you're not getting burned out breeders or poor producers.

Also, buy from people producing rats as feeders, not fancy lines. The difference in litter sizes, growth rates and overall size, are like night and day. (A good feeder breeder will be forever breeding towards increasing these values, fancy rat breeders will not, they want specific looks! not feeder factories lol)