Most freshwater stingrays belong to the Potamotrygon family, and come from South America.
There are many species, which vary significantly among each other. However, info is limited. I've researched a lot in the net, just to find the same stuff over and over, and sometimes contradictory info.
I decided to go in the printed way: I bought two books about FWstingrays anf found nothing but the same I already knew.
As for species I remember 4: P. reticulata, P. Motoro, P. histrix and P. Leopoldi. You can find P. Laticeps, but thet's the same as reticulata.
Beside those 5, I can recall other common names as china, apple, tiger, otorongo, and henlei stingray. I know there are more, but can't recall them all.
If you are planning a stingray in your aquarium, make sure you have a big tank (the smaller species will be 12" wide + tail. others can pass 30"). Also, these fish are carnivourous, so you'll need to feed them meaty food. They won't take flakes. Carnivourous also means lots of waste, so a heavy filtration is required.
This link helped me a lot when I got my rays:
http://www.geocities.com/Heartland/Meadows/5590/ray.html
You can also visit
www.aquaticpredators.com