Potamotrygon Hystrix come out of Brazil and Brazil currently has a ban on export of ALL rays, so best of luck finding one and if you do it is going to cost a lot.
-That said, what you may find listed as Hystrix now will usually be Potamotrygon Humerosa--they get as big as Motoro.
Potamotrygon Scobina is the smallest of adult rays and cost a lot and is also from Brazil.
Followed by Hystrix and "Retics" and P14 (P14 cost an arm and a leg and are also from Brazil.
Potamotrygon Reticulata - max at about 14-18'' females getting larger than males and rumor has it they may get larger. But, they are the cheapest ray and usually easily available.
But I'd suggest learning a ton more about rays before buy.
And not to discourage you, but I think you were the one having trouble cycling and clearing your tank, until you get your tank keeping skills down pat and you really know your stuff, I'd suggest not getting a ray.
I'm not saying this to discourage you; I'm saying it to not discourage you, because down the road you'll loose the ray and then end up giving up on them thinking they are too hard to keep.
If you have any questions on keeping rays or water quality, please feel free to PM any questions.