It's probably annoying to hear this again, but it really does pay to buy them when they're on sale and stock up. Unless you're a flavor-of-the-month tire buyer, you'll always use them, and they're not going to get any cheaper. I'm set for the foreseeable future on tires and tubes.

PBK has always been my source- prices (can be) really good, shipping is reasonably quick, and service is good.