You can look for breeders on AKC's website to get you started.
http://www.akc.org/classified/search/landing_breed.cfm
Though if you are looking at $500 or less you might want to go to a local rescue group instead. A properly bred and cared for purebreed can go for over $1k. Most ones that are cheaper are typically "Backyard Breeders" that do not care for the dogs nearly as well as they should and do not screen the parents for hereditary diseases. To give you some perspective, I got my Lab from a reputable breeder and he was $1250.
Not to say you won't find a good breeder in your price range but just be careful as to where you get the pup from. Go visit the breeder and see the conditions the pups are raised in and meet the mother to see how she looks. As to see vet records of the mothers care during pregnancy.
Reputable breeders will also offer a health guarantee for 2 yrs, and will always take a pup back for any reason whatsoever. And most important is go to a breeder that will ask you a lot of questions and really care about the home the pup is going into. It shows they care about the dog and not the money.