Believe it or not, a breeder is probably the best way to go. Pet shops are notorious for providing bad advice to new owners and often keep their amphibians in awful conditions. Online is also dodgy, because there's a chance they'll send you a sick/injured animal, or it may be DOA (dead on arrival). I don't know of any animal shelters that take in frogs, but that could be an option.

But in my opinion, the best thing to do is find a trusted breeder with a good reputation. They'll make sure you get a healthy animal, and generally will go to any lengths to make sure that the animal arrives safe and sound. There are several reputable breeders on this forum, why don't you sniff them out and send them a PM?