You really don't want to buy old breeders (they are usually used up...that is why they were sold off). Most will buy young rats and raise them up. This way you know what they ate and how healthy they are. Not to mention interacting with them makes it easier when they have babies to be able to look at and mess with the babies.

As for where to get them...I hate pet stores (the rats are usually overly imbred and sickly) but not everyone has access to anything else so sometimes you just have to take a chance.

Better options: any reptile shows near you (usually you can pick up rats from there to raise up), rat breeders (ideal but if they find out you have snakes they will go postal on you and refuse to sell them...not to mention they usually have a bunch of hoops you have to jump through), people from this site (or others) who might also breed rats. Where do you live?

Hope this helps,
Sean