The type of fish you breed is YOUR choice.
Can you breed a petstore betta? YES... but do you want to?
This all depends on your reasons to breed. If you're breeding because you want to see and raise a spawn, then it really doesn't matter what fish you use.
IMO, if you're spending all that money and effort breeding and raising 100+ fry to adulthood then, why not start with the best pair you can get your hands on?
I'm not talking about the show quality of the fish but about your possible "ideal" fish. Do you absolutely love dragons? Then get a pair you really like and use that one instead.
Petstores have some beautiful fish and if you look really well you might find one that's breeding-worthy but the problem is that you cannot predict the outcome of the fry unless you know some of the genetic background of a fish. An honest breeder can provide you of this by showing you pictures of siblings, parents and other fish in that line. This gives you an idea of what you can get when breeding this is very important for ME.
Anyway, here's a good website to start off: http://www.flippersandfins.net/BettaBreedingArticle.htm
This one has also a lot of good info: http://www.bettasrus.net/bcorner/index.htm
Best of luck and keeps us updated!!