Yes, they have been banned in most area's. You still see the odd one on Ebay, but, now that they are banned the prices are more than you could buy them for "new"
I looked at buying one myself, then checked out a few YOUTUBE video's of them crashing ]-:
I dont mind having my fat ar$e thrown out of a tube while ON the water, but, falling 30 or 40 ft BEFORE wiping out is a little too crazy for me.
Good luck in the search though, I found on Ebay that "kite tube" didn't always find results, so, try "Manta Ray" and "WEGO" KITE TUBE
Michael