The less common livebearers come from other hobbyists. I go to my club auction and one of a nearby club and pick up livebearers at most auctions. I also attended the ALA convention this year and got a few nice livebearers at their auction. There are similar clubs in the UK such as the BLA so that would be an option there too.
The only special care for the less common fish is that I don't mix them with other livebearers. The unusual thing about the goodeids is that most of them do better if you unplug the heater in their tank. The normal daily temperature variation and seasonal temperature variation seem to make for healthier more prolific fish. There are also some goodeids, Skiffia, that can be a bit of a challenge because they do not do well at normal room temperatures, they need to be kept cool. Most of my fish are as simple to look after as a swordtail would be.