Try wheelchair hubs - they're essentially bike hubs but intended for single-sided mounting. However, a quick search doesn't seem to show up any bargain wheels or hubs....
When I've looked into this, I've set up an
Ebay search and had it running for some months before something has turned up - usually from someone else who'd started a DIY recumbent trike but given up.
You could use regular wheels, supported on both sides; I've built trailers this way. Round tubing is cheap and doesn't have to be too thick-walled to be pretty strong - and you can bend it with plumber's pipe benders to produce nice curves (you may have to heat it up significantly first, as it's rather stiffer than the copper pipe plumber's benders are designed for). Then all you need do is cut out and weld/braze on some dropouts from suitable sheet mild steel.
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