They're freely discarded in junk yards, I've collected several sets when removing wheels from trucks and just laying around. This is probably by far the cheapest too.

Can you use your old original clamping plates, I did on my GMC with newer 19.5 wheels, nothing wrong with that.

•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-