I was one of the people who built a 29" Onyx cruiser. I immediately got a flat. I cant find a 29" inner tube with a stem long enough to go through the rim.
The walmart here sells 29" inner tubes and they have schraeder valves, cost about $8 each (which reminds me that I should buy a couple). I'd avoid presta valves, more hassle then they're worth unless you're racing.