I would agree about getting rats from a breeder that breeds for health and temperament. You can get a list of reputable breeders from the nfrs by emailing
breeders@nfrs.org. These rats have been bred responsibly and any unhealthy lines are never sold on. Also these breeders will always take the rats back if needs be. The only time you will get problems with breeders regarding health etc is when you buy from a backyard breeder.
Aside from this health can never be guaranteed but I would much rather buy from a breeder than somewhere that supports the pet farming trade and who often hardly have a clue about the pets and often missex. The amount of people who hav ended up with babies after pet shop buys is ridiculous. Anyway good luck and ratties are lovely pets.
EDIT: Also forgot to mention, resues are also great places to get rats. The older rescues are usually better for more experienced owners as they can sometimes be difficult to handle/not very tame. However some rescues have babies in-one close to me has a mummy rat and 13 babies so its worth looking around.