Since you are in Kentucky and would have to have them sent to you, you might consider the wheels from Fairview Country Sales - Millersburg, OH - 330-359-1501. Merle, Willis and Jr are great folk to deal with. They don't have a website, but do send out catalogs. The wheels and tires come in 20, 24 & 26" sizes.
I have the heavy duty wheels, with no air tires. It did change the ride a bit especially on the ez entry one with the little springs under the seat. Not quite as smooth, but worth it to not have any more flat tires! For that particular cart, I also had to upgrade my axle - a little wider and heavier duty, but again, soooo worth it. My other cart is a sulky style with no axle...
Haven't used the cart (s) much since getting them back - have been doing mostly ground driving babies and working the pairs to other vehicles, LOL. Just enuf to notice the difference in "ride" and to realize athat I still like them since I won't have any more flat tires. I don't expect the metal to waffle - but it probably still could if torqued wrong - they are still essentially bike tire/wheels.