Personally, I would take a look at both.
Quite often, a breeder is the better route to take because they know the rats' "family tree". Plus they could be considered somewhat of a "rat specialist" since they deal with rats specifically. Pet stores deal with a bunch of animals and don't focus on specifics. Quite often pet store workers don't know what they are talking about and give wrong information. (This isn't in all cases, but it does happen quite often)
However, you can get really healthy, really happy rats from pet stores.
Also, if you check out both, it gives you a wider selection of rats to choose from. ;)
Here is a good site to get your started:
Ask lots of questions. Do some research beforehand. Make sure they have been properly cared for.