People usually don't import them because the small ones die so easily, and Tigers are sensative in the first place. They are also one of the larger species or rays, compared to histrix and motoros (often named 'teacups') so you have to keep in mind their live young will be proportionatly larger than that off the other species often labeled teacups.
Young rays like that need to eat 3-4x a day, and they can't handle the day or two of 'purging' before shipment. They stress from the overseas voyage, only to never eat again, curl, and perish.
The smallest I have seen, healthy, was an 8-9" one I believe David Webber had. They go quick at that size, too..