I'm pretty sure the only means is the NPC bought breeding horse birds. Technically, they are all purebred anyway in that a chocobo is only a chocobo in this game (I get what you actually meant). I haven't bred them yet, but I'm assuming you can only stable 1 bird at a time (not including a breeding one). Maybe they'll expand it someday to allow this, so that you can breed them once then start working on your own, assuming they don't change the base 2 star starters.