I have gotten rats and mice from local pet stores that have died a few days later. The ones at Petco seem to be more for pets so they are usually the ones in better shape. Local pet store just buy anything to sell off as feeders.
I got rats from one source and had 5 die in 5 days. So I never went back to that place.
I am not sure why they died.
I can't take the smell of dead rats and mice, so my girlfriend has made some money off of me. She will get them up if I pay her!