I am finding out the same thing you are. Parts for the model 68 is getting harder to find. The first thing you need to do is measure then. Shaft size, outside diameter, and width. You should be able to buy them from anyplace that sells bearings like NAPA. Some times you can find a bearing number on them and they can also go by this.
If that's a JD Model 68 rear engine rider you're talking about, go to a JD dealer. I replaced one of mine last year and the new one was for a later model and used a spacer.