Just to let you know how the "production" wheels are done.

The hubs are nothing more than a rim that has a 4" deep through hole. This hole diameter is about .5" larger than the shaft/axel. Some delrin rods (1/4" dia) are stuck in between the shaft/axel and the ID of the hub to act as bearings. This hub is all Aluminum and so is the shaft/axel. Both are anodize so less prone to corrosion and very light.

Basically, the wheel itself is the hub. It's very simple. But, as you noted the trick is to find them somewhere.

If you are looking for trailer type stuff try Northern Tool this might be a good place to find some plastic hubs.

Good Luck