For the past two or three years, I have been getting much lower prices on consumables (tires, tubes, chains, cassettes) from several British wholesale outfits than I can find anywhere locally or even mail-order in the US. Most of these companies offer free or heavily discounted shipping also. Try Ribble, Probikekit, Wiggle, and the like.
In the US, Nashbar and Bike Tires Direct; Jenson and Universal Cycles are also good for low end (i.e., commuter bike, not fancy road race bike) parts.
