I know that a local petsmart said they would have chinchillas when it got cooler but they never got them in, at least to my knowledge. The preferred to sell guinea pigs. Another petsmart about 5 miles away seemed to sell them year round. The only other store I know that sells them also sells them year round.
Obtaining your chin from a pet store is a very controversial thing and it really depends on the store whether or not it's a good idea. But unless you have a relationship with the staff there that is in charge of the process there's know way to know.
Some stores care about their stock and are fairly good at maintaining a chinchilla but I think most do not, at least, from my cursory observations. The problem is that chinchillas typically obtained from a store are not going to be as tame as a hand raised chin from a breeder. They get weaned and maybe are hand raised going out, but if all they know is a small cage with no interaction for several months, it's not always possible to bring them back because they just want to be left alone. Chinchillas are still pretty wild and must be handled regularly.
If you have a breeder who is reputable and loves chinchillas chances are they will be open and honest and answer any questions you have. So you can know what its background is and even see its parents for yourself so you can verify the kit is healthy. A reputable breeder uses best practices for selective breeding so your chances of a obtaining a pet with a genetic problem are much less which is a big plus.