Moving rats can stress them, and a lot of feeders are mill-bred which will also run you into issues. Petsmart/Petco get their rats from the same distributor, Petland often uses local dealers, and you'd probably have to ask the others who they used to get the rats.

I know Petco/petland do have a 'return policy' if a rat dies within 14 days (or similar, don't know exactly which.). If a place has sick rats, assume they're all sick (or at least, been exposed to the illness.).

I might try finding a local rat breeder off of CL or similar and seeing if you can't get some quality rats from them. It's weird that all have died so quickly. You should wait until the females are at least 4-5 months old before breeding them, breeding them too young can result in difficult labor.