I was just down at the local "Northern Tool and Supply" store, and noticed they have bicycle-style wheels that could be used for bike trailers. These are 26"x2.125", 20"x2.125" and several smaller sizes. Wheels have big hubs, heavy spokes, bearings already in place to take maybe a 5/8" or 3/4" axle. I haven't used those, so can't vouch for durability or capacity, but they could be just the thing for some trailer applications. They also sell some lawn carts using similar wheels. And a bunch of go-cart type wheels and tires for that matter.