tccole,
I have actually seen little flounders for sale at one of the LFS I go to......but.......I wouldn't buy one. They get very large very quickly and live much of the time buried under the sand....so.....you'd need a very, very large tank with a very deep sand bed.
Then the bad news is they also would eat many of the smaller fish you'd probably want to keep in your tank like clowns or gobies or grammas.
JMO, but I'd pick a different species of fish to keep.
Jim