I'm with
@emeraldking . For years, I kept and bred Xiphophorus. My experience says forget asking stores. The fishfarms have a way of getting 3 species of a swordtail, for example, and immediately crossing them to make a commercial hybrid. They never seem to value the difference between a species and a 'strain'. They do the same with rainbows.
So that drives you back to a small number of people who keep wild type livebearers. For me as a Canadian aquarist, I know of half a dozen people in my entire country. For you in the USA, the ALA will be your friends. Your larger population base is a luxury in the hunt.
You may see ALA people selling on AquaBid, but this is the off season.
When I had a lot of wild types, I networked, networked and then networked some more.
One of the funny things in our hobby is that I have never once found a Xiphophorus maculatus in tanks. I've watched them in Belize, in a river, but the supposedly common platy is the rarest of the rare. My local stores have tons of platys/swordtail/variatus hybrids, some going back to crosses close to a century ago. But the fish as you'd find it in nature is one I just have never gotten my paws on. I have the closely related X milleri here, but maculatus is a holy grail fish.
I got M picta via Germany, once. I've had wild guppies, from a Trinidadian friend. They were great. I lost my montezumae a few years back, to a mystery disease, and would love to see them again. A friend had gone down to the ALA convention, and I did the 7 hours round trip to get my original pair, from his fry. I had them for years. You have to really be willing to hunt for these fish, unless by posting your question, you've networked enough that someone sends you a PM, and you're on your way!