700c Schrader tubes are fairly common now, most box store bikes I've seen with 700c wheels use Schraders. Even my nearest WalMart, which is in a town of 4,000 people, carries 700c tubes in both Presta and Schrader valve, they even still have 27" tubes and tires. I wouldn't use them on anything I planned to go very far on but good to know in a pinch, I buy most of my tires and tubes from NASHBAR, I've had pretty good luck with their house brand stuff.