Gidday, A link to a recent similar thread…
http://cs.trains.com/mrr/f/88/t/218319.aspx
Looking closer at your wants how about something like one of these…
http://www.harborfreight.com/automotive-motorcycle/creepers-seats/mechanics-roller-seat-3338.html
While not sure about costs purchasing individual wheels, looking at your bio ,perhaps custom building your own to suit Your requirements may be the best in the long run.
Cheers, the Bear.