As far as I can tell, there are 2-3 in existance.

One just came out made by Gyration. "Perfect" concept, but a few critical flaws. Check out my review of it on Amazon
Another is made by Niveus, but available only in EU unless you can find some on Ebay. And there are a few Niveus clones that are no longer made. However, if Gyration was made with less bugs, Niveus wouldnt even come close

HTH