buy weels with hubs bored and keyed to the axle size you want. this is done either by buying a bike trailer from retail then chopping it, shopping hard on ebay/amazon, worksman direct, bike shops, etc. read more
Bike wheels (or rather their hubs) are generally completely different to trailer wheels - trailer (or wheelchair, etc.) wheels fix rigidly onto a rotating axle, with external bearings or bushings between axle and vehicle body, while bike wheels rotate around a stationary axle with bearings inside the hub which are designed to have an equal(ish) load on either side. read more
use a standard bicycle wheel with a thin through axle, supported from both sides. It's possible to make a really simple trailer this way that will carry quite a lot. The disadvantage is that it's wider than a single sided mount for the same track width (how far apart the contact patches are). read more