Never seen one for sale.  But who needs to buy?  They're just so easy to make.  As I said, not my own idea - can't remember the first time I ever saw one.

I seem always to make them as I need them - I can never find the last one or someone's 'borrowed' it.  And if I use one in someone else's house I'm rarely given any choice but to leave it there.  But it only takes 5 minutes to make another - given a length of wood and a nail almost anything will do for a handle.  I've made them from both wooden and plastic 12" school rulers with 6mm nuts&bolts as both handle and pin,  and I've managed at a pinch with a knitting needle with both ends bent over (though the knitter wasn't best pleased).

I've even made one out of a piece of square plywood with two handles and two pins to hang a heavy picture that needed two hooks.  Once the principle is grasped, the rest is easy.  I'm sure there must many variations.