Why not just get PUL? It's not really that expensive as yardage goes...
If you do use nylon, I would recommend using coated nylon because it's much more water proof than regular rip stop nylon. It's really important to keep nylon covers out of the dryer too. The dryer will break them down faster and they will lose their water repellant qualities.
I used dermoflex for my first diapers because I wanted a Canadian source for fabric. I bought it from Wazoodle http://www.wazoodle.com/cgi-bin/cats...abrics;Barrier
It works quite well though it's a woven so no stretch to it (the diapers won't be worn quite as long since they don't have that stretch) and it did compression wick a bit (thus meaning I had to make sure I had enough absorbancy in them) but they did work. I imagine the coated nylon they have would work fine too (you can probably find it locally too) Wazoodle has good PUL now though (only white but the other colours will be there very soon) so I'll just get the real stuff there. PUL makes really nice diapers. After I made my first one I swore I'd never use a 2nd string fabric again!