These are the same 10" pneumatic wheels we supply with our garden wagon. We offer them separately for your homebuilt projects such as custom carts, dollies, wagons, etc.
The central bushing rides on ball bearings and accepts a 5/8" axle or bolt. Each wheel is rated to carry 300 lb, so four of them together will hold more than half a ton.
For extra durability the tires are tubed, not tubeless.