this is what my friend said since his the one that i know who knows about such kind of info.
here in malaysia, we didn't have any distributor for that. however, you can contact the 'taukey' aquarium at shah alam lake to help you to buy it from thailand. but, if you are looking for super size freshwater stringray, then you might need to catch it yourself there and bring it here also by yourself.
sorry because could help you much but just some idea, maybe you can surf the internet and search for exotics fish distributor.