I just ordered the Greenspeed GTR PLans last week. They should be here sometime this week. Any of you know where to buy tubes from? Also, How to you converyt a normalfront hub so that it will fit on one side? I found a Diatech 20mm MTB disk brake hub that would be great but I don't know how to make it one sided. Please, give advise.

I've ordered 4130 tube from http://www.aircraftspruce.com/main.php

Your on the right track using 20mm axel hubs for the front. Take a look at the Organic Engines web sight http://www.organicengines.com/Index.htm look at the photos for the take apart Troika, http://www.organicengines.com/Troika/Saw_into_three_bits/index.htm

Its easy to use the hubs as single sided.