Try attending an auction by your local fish club. My own fish club held an auction just yesterday and as usual I acted as one of the "runners" carrying people's fish to them. People attend these auctions from far away and often have fish you will not see in any pet shop. I saw several sets of moscow guppies, half blacks and similar guppies at that auction. I also picked up some nice Xenotoca eisenii, red tailed goodeids, and some Chapalicthys encaustus, barred goodeids, while I was there. If you attend an auction, volunteer to be a runner. It gives you a close up view of the fish in your hands and will let you sort the best from the rest.
If your local fish club does not have auctions or sales, talk to the other members at a regular meeting. You may be surprised at what they have that they don't mind parting with.