My PetSmart always carries them. Don't depend on them too much because sometimes you'll get bad batches that will produce a very small amount. For the most part, they are useful for starting colonies. Buy or make your own medium and use the flies from the vials.
PetSmart and PetCo carry them. But yeah, call to make sure they're in stock because sometimes they aren't. The kind they have is D. melanogaster, even if Petsmart calls them hydei.