Well, since they come from the Americas, I guess they must be around somewhere!
The question is where do you buy them. The first step is to ask a mom-and-pop tropical fish store if they can order them. Shops have lists from which they pick stuff for their stock each week. The better places will take orders.
Failing that, try and find a tropical fish club in your neighbourhood. Most states have at least one. Even if the place is too far to visit, they may know someone breeding the fish in question. The British Livebearer Association regularly has meetings and auctions from which you can pick up all kinds of good stuff. I have no experience of the American Livebearer Association so hesitate to comment further on that.
Finally, you can mail order fish or buy them on places like Aquabid. Personally, I'd see that as a risk because you can't inspect the stock before you buy it, but many people have done this and been pleased.