This is the only place I could find as well. But they are out of stock.here's an old thread.
It's pricey in the EU, if you can get the kit number and search?
This YT vid really shows how well it works and installed.
This place ships to the USA.
PTO shaft conversion kit for Shibaura SU1100, SU1140, SU1300, SU1301, SU1341, SU1500, SU1540 Japanese compact tractors | KELETAGROThis kit is serving the transformation for the Shibaura S series Japanese compact tractors, like SU1100, SU1140, SU1300, SU1301, SU1341, SU1500, SU1540 type tractors, which originally do not have standard, 6 ribbed, 1 3/8" (38mm) PTO shaft. With this kit the tractor's PTO will be conversed to a...parts.compact-tractor.eu
Thanks for the help I was able to get in touch with a North American dealer for their products.