Try to avoid getting rats from unsafe places. This is really hard to do because you can never be sure what is happening in other ratteries or pet stores. I have no beliefs that private breeders are always a better or worse source than pet stores.
Most stores get their rats from some breeder anyway.
There are breeders who have a reputation for having healthy rats and that is a good place to consider. Getting Rats at Shows and Other Gatherings. Sadly, while this is the easiest and most fun way to get new rats, it is the most dangerous. All rats there will have been exposed to all the other rats in attendance. If you are picking up rats that have been reserved for you, you can try to make arrangements to get the rats directly from that person outside the show and put them in your car.