Not sure I'd call 19.5" tires "semi tires", a little small for any semi truck I've seen. However they are quite popular on medium duty trucks, especially the low profile ones.
Look at the 10 lug rims on many Chevy P30 chassis. They are 19.5", and with a little work should fit. Check the fit over the brake drums before buying any.

http://www.stovebolt.com/techtips/wheels/

•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-