I would look at a shop that does front end alignment on I-beam type axles, they either have the shims, or have a suppler and can get the size you need. They may even have some advice.

I found the caster shims I needed on my milling machine.
http://rides.webshots.com/photo/1484200068080251109FlXuOW

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-