Burley push button wheels and axle?
If what you really want is Burley hub/attachment guts, just ask around for someone getting rid of a Burley trailer with rotted out or missing fabric. They've sold a lot of trailers and many of them sadly get put outside to languish before eventually getting thrown away once they get nasty.

Alternatively, Ali Express has a lot of the approximate kind of stuff you're looking for. I don't know if you know the term "stub axle," but that is broadly what you're talking about, an wheel axle like a car or a wheelchair where the support is only on side.