I've done my research on rats, and I've heard it's not good to get your pet rat from Petsmart, but that is the only choice I have at the times. I'm planning on getting two females. I went to look at my local pet shops and the best place to get them was Petsmart they were all females paired together. I did not ask if they were from the same litter because I was not buying them that day. I have decided so far to get them from Petsmart. I believe its not super bad to and I would like to hear other people opinions. Not the really bad ones like don't get them there I would like a comparison of breeder and pet store.
Depending on where you live there may be a third option that you have not mentioned. It would be my first choice.
In Pittsburgh we have a local rat rescue Pittsburgh Rat Lovers Club and Rescue check if you have something similar locally.
My second choice would be your local shelter, many shelters take all kinds of pets. They may have rats waiting for their forever home, or be able to put you in contact with a local rat rescue.
The basic problem with pet stores is that some animals come from breeder mills. You buy one and they breed a few more, some of which find nice new homes and some don't. The quality of stock in a pet store is often different in different areas, so a store specific answer would need to take your location into account.