It really is unobtainable, and does wear inside. It's going to turn around the axle, in spite of the fact that it goes into the bearing. In fact I think its movement around the axle is freer than the bearing movement. In addition, it's very soft steel. Plus it gets beaten on to remove the bearings.
I would try to copy an unworn one and put them in an ebay store and include the Yamaha name and part number in the listing (90387-20166-00 COLLAR). Maybe mic out the ones you have and go with the smallest internal diameter if you can't find what it was supposed to be new.
90560-20135-00 SPACER (#7) seems to be unobtainable as well.