Yep, sounds like Motor Wheels.
They are not rare but not all that common in my opinion.
Best bet to find some is start walking through junkyards, that's where I see them more often than not. Last truck I saw with a set was south of Hutchinson KS in a junk yard...

Good luck,
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-