That looks like the bolt pattern of the old, 3/4 ton pickups from the 1940s, 50s and 60s (when 16" wheels were common, original equipment). So, perhaps an advertiser in Hemmings Motor News (hemmings.com) or maybe Custom and Classic Trucks magazine.
Perhaps a Space Saver spare tire from a more modern 3/4 ton pickup would fit.
Try Golden State Truck Parts or even Chevs of the 40s.