I bought mine (exactly the same one in the pic) many, many years ago on a PC convention. I used it on my Amstrad Mega PC, a combined Sega Mega Drive/Genesis and 25MHz 386 SX. So you can count back how long ago that is. So I don't think they're still available now, except perhaps on eBay.

I've also seen one once that was specifically for the numpad. It was a stick on a suction cup that was supposed to be stuck on the 5 key, and then four fingers extending from it would push the up, down, left and right keys when you tilted the stick in that direction. More elegant, but less sturdy.