Probably the most widely available 27" tire in the USA is the cheap Bell branded 27" x 1 1/4" kevlar-belted blackwall tire sold by Walmart:
http://www.walmart.com/ip/Bell-Sport...-Tire/15188809
Every Walmart I have been in stocks these. You can find these or similar cheap 27" tires at hardware stores, general stores, etc.
As long as Walmart continues to stock 27" tires and matching shrader-valve tubes, there are large areas of the country where the most readily available tires are 27", not 700C. If you are touring, it's a lot easier to find a Walmart than a bike shop.
Not all Walmarts stock any 700C tires or any narrow presta-valve tubes. Most of their tire and tube stock is wide 26" tires and matching shrader tubes for cheap mountain bikes.
I can't find them in their online catalog, but I believe Canadian Tire (a ubiquitous and uniquely Canadian store that sells both automotive and sporting gear) stocks some similar 27" tires.