I've been getting some nasty flats on my LHT's rear wheel. I've taken the tire off, found nothing, put a new tube in, only to keep getting flats. Any suggestions?

Also, I'm a bit concerned b/c I can't really find tubes (26 x 1.25-1.5 for my 1.5 WTB tires) at any of my local shops. I plan on riding across the country (some day) and I'm concerned about being able to find tubes along the way. I've got presta valve style wheels which seems to complicate matters. Any thoughts? Can I run a 650 x 1 on this, or is that too risky?

I love this bike and am hoping that this tube issue doesn't complicate matters for me.