It would be a very simple project for a machinist to fabricate one of those spacers from a block of steel and size it to whatever axle end play was needed. I'd probably choose to case harden the surface of the metal with something like "kasenit" before grinding the thrust surfaces to the final dimensions. Case hardening produces a hard surface about 1/16" deep on a piece of soft steel. That would be ideal for a part subject to surface wear that is still required to remain "tough but flexible" on the inside. Tool steel could be used for a project like that, but it would be a little too brittle if it was hardened all the way through.
Jerry

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