Depends on your definition of minibike; if it's what most members of this forum consider a minibike, I'm pretty sure the answer would be none. The smallest Monster Moto is 79cc; the smallest Coleman is 98cc; they are the only small minibikes available new, that I'm aware of.
There are 50cc scooters available, even a few 2-strokes from Genuine and Adley. Yamaha and Lance have 4-strokes, plus there are a bunch of Chinese fifties.
There probably are 50cc pocket bikes, but I'm not sure.