They are called spacers and should be available from the
manufacturer. They purchase them from a number of sources including sources outside the country. They can be made from a variety of materials depending on the application.
Most are polyethylene or similar material. I assume you are speaking of ball bearings but the same is true of cross roller bearings as well or even 3 row roller bearings.
They may be inch sizes or metric to match with their rolling element sizes. Some sizes fit both inch or metric and are interchangeable.
