I found a large pair of geta for my nephew (size 11) at Soko Hardware in Japantown, SF last year. But we had to lengthen the strap holding the thong on them so his feet would fit. We removed the metal cap on the underside, removed the strap, made a longer one, wrapped around the thong, pulled the strap through the hole, tied a knot, then nailed the cap back over the knot.
He wore them almost 90% of the time we were at AX last year, even when we went to Disneyland. Still wears them.