The good thing is that when you are looking for Pseudomugil or Threadfin rainbowfish they almost certainly will be good stock. There are only a handful of people who are breeding them in the US, and the bloodlines are good. That is unlike some of the more readily available overbred LFS species that can sometimes be suspect.
Both species stay quite small, and most likely would be good with a solid shrimp population. Of course there is only one way to find out. With my tank of larger Bows they would be nothing but a quick snack.
As others have said most of the best breeders post availability on Aquabid. But keep in mind there are also some "shady" ones who buy from a wholesaler then resell them. So it's good to know who are dealing with. If anyone has questions post here or reach out via PM and I will give you my thoughts.
One issue on Aquabid is that they are usually selling a minimum of 8 at one time. And it's hit and miss depending on who has what available. Roseline17 (Marcel) is one of the top breeders and he usually has some interesting selections including some pretty cool Rhad's which are also small and colorful. If you want a mix of different ones you can reach out and work out a deal.
Many well known good breeders also sell through Dansfish and Getgills. They don't carry junk fish and can be trusted as a good source.
Then you have some more commercial places like Imperial Tropicals and the Wetspot. Over the past decade both have been working with breeders to bring good blood lines into the hobby.
So in the scheme of things right now it's easier than ever to find well bred Bows with good lineage. Keep in mind shipping adds up, so it's best to stick to one source if you can.
I hope that helps and reach out with any questions.