Turntable Bearings
Shown below are photos of the 9-inch round turntable bearing from McMaster-Carr. It's rated to support 750 lbs. Should be enough!
These bearings are attached to the wooden spinner parts with ordinary wood screws. There is a large hole on the bottom of the bearing to allow access to the upper screw holes for assembly (see photo below). When the bottom is rotated, the hole allows access to all the top screw holes for assembly.
Here's the edge view of the 9-inch turntable bearing showing how thin it is.
Shown below are photos of the 6-inch square bearing which might be used instead of the larger one above. Sometimes these are found in local hardware stores.
Even this small 6-inch square turntable bearing should work for a spinner, but be careful that the load (you!) doesn't overhang the bearing race. Weight should be positioned close to the center of the bearing. Excessive overhung loads (weight positioned significantly outside the round bearing race) can damage the bearing by prying the races apart. This smaller bearing is more likely to suffer from the effects of a mis-positioned foot, but it should be OK with a little care during use.