Pima are semi readily available. And there is JohnPTC and ArapiamaG that both have very large ones but both have 10K gallon or larger tanks.
Mekong cats are not available due to being protected/endangered.
White Sturgeon I have seen these in home aquarium (pics at least) but even if you can get your hands on one it will never get that large... at least in your lifetime they live forever and grow slow.
Chinese Paddlefish have not been seen in the wild since 2007 they are thought to be extinct but it's not sure.
Giant Stingray little is known about these and to my knowledge they have never been available to own privately. and I'm not even sure that any public aquariums have any.
Alligator Gar Readily available native U.S. species. Can be bought for under $30 but get large and mean
Blue Catfish readily availible
Giant Carp Few have been for sale here but are hard to keep alive from what I've heard. and due to max size people shy away from them.
Most of them can be had in the home aquarium but due to the large size they all get it's not really a attainable fish b/c the housing requirements.
