There are no 18" mag rears that swap right in, at least not that anybody has found or mentioned yet.
The desirable alloy spoked flanged wheels have turned to gold, as you probably noticed looking on eBay. You might do better finding some locally. My buddy scored a pair last year from a local cycle salvage place for about $50 - for the pair. I was amazed the guy sold them so cheap, I thought he meant for one wheel. I don't like spending more than $20 to $30 on a wheel because new (stainless) spokes are going to add about another $80 to that - and they all need new spokes, lol.