Agree with s2mikon. Switching out to a single piece wheel is the way to go as finding shops willing to work on multi-piece wheels is getting harder, if not impossible. I made the switch to 6-lug single piece wheels a few years ago on a truck I had and it was not easy finding the wheels. You have a lot of junk yards in your future. Your tape measure is your best friend on this adventure. Measure your lug pattern and the diameter of the center hole. You appear to have stud-piloted Budd-type wheels (most of those 6-lug wheels were, but you never assume when junk yarding). You will need to find the same thing -- stud-piloted wheels with the exact lug pattern. Look for similar vintage school busses and Ford U-Haul trucks from the 70's.
Stud piloted wheels have the conical shaped lug nuts. Hub piloted wheels have flat nuts, usually with a built-in washer. IF you find aluminum 6-lug stud-piloted wheels in your lug pattern (which is the equivalent of finding Bigfoot holding the Holy Grail while riding a unicorn -- but you never know ...), just check back with us as there are some important aspects to know when switching your set up from steel to aluminum wheels. I doubt aluminum 6-lug Budd wheels exist but who knows what you'll run into out there....
In addition to junkyards, also put the word out among the big truck tire shops and antique truck clubs near you. You never know who has what just laying around.