They are custom made, you turn them out on a lathe.
I have never seen them offered for sale.

Before you decide on 19.5" wheels first test fit one on both the front and the rear of your truck, don't need the wheel center bushing to test fit.
If they fit over your brakes then continue as planned and have the bushings made. If they don't fit then reevaluate your plan.

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-