Gas canisters are becoming more widely available, I've seen canisters in my local Wal-Mart that are made by Coleman but would fit an MSR Whisper Lite or Snow Peak Giga Power (I have a Giga Power, it's a good stove).

REI has a note on their website saying that they can no longer ship butane canisters due to DOT restrictions on flammable items. They won't ship Coleman cartridges, and they don't show MSR or other cartridges on their website. It looks like the rules have changed for that sort of thing, so I would check with the Post Office or UPS before planning on shipping gas cartridges anywhere in the US, surface or air.

Having used a variety of fuels, I've settled on alcohol (Trangia cookset) for warm weather travel like cycle touring, and white gas for backpacking/mountaineering stuff. I can get alcohol in any hardware store and probably wherever I cycle in Europe, and it's just about as easy to use as a gas canister stove. For backpacking and mountaineering, I can buy a gallon can of white gas in Wal-Mart and pour what I need into my fuel bottles for each trip. I've had too many hassles trying to find a gas canister that would fit my stove, and I don't like having to dispose of all those empty canisters.