The problem is that a shop won't know the parentage and even if someone has bred their own, if they got either parent fish from a shop they can't know either. The only way to be 100% sure is to get fish from a registered keeper. As you are in the US, google swamp river aquatics. That site belongs to a man who does have pure endlers. I don't know if he is selling any endlers at the moment, you could always contact him to ask. If he isn't selling, he may know someone who is.