In addition to the original habitat of the guppy (see write-up above, from Percepied) and the aquarium, guppies (and to a lesser extend neon tetras) also can be found in rivers and canals near thermal power plants in Europe, but I assume in other areas around the globe with a moderate climate too.
The used water of the power plant disposed in the rivers is consistently a few degrees higher than the normal water temperature, providing a acceptable environment for the guppies not only to survive, but adapting themselves to the new situation to breed and flourish. The guppies can get there because of the people who don’t want the guppies in the aquarium anymore, or just some but not all: they flush them through the toilet.
My dad was one of those persons who did that too, “giving them back their freedom” he called it...