You can buy the cannisters in quite a few places. But, there are some gaps. The info below is 2003 vintage, so beware that things change, etc. You can buy cannisters in:
Idyllwild (outfitter)
Big Bear Lake City (ditto)
Agua Dulce (car from Saufleys)
Kennedy Meadows (had in 2003, but you’d want to check)
VVR
South Lake Tahoe (outdoor store on the way to buffets)
Ashland (outfitter)
Bend (ditto)
Those are all that I know of. I’m pretty sure Hood River (from Cascade Locks) has them. Leavenworth (accessed from Stevens Pass, opposite direction from Skykomish) probably has a climbing store with them. Tuolumne Meadows did not have them in 2006.
The big gap is from South Lake Tahoe to Ashland. Depending on your hiking rate, you’d have to carry at least 2 out of SLT to get to Oregon. However, rather than staying in Castella or hitching to Dunsmuir, instead hitch to Mount Shasta City, which is a way cool town and has an excellent outfitter. That breaks up the stretch nicely.
So, you should be able to get away with only a couple of mail drops of cannisters on your hike.
Suge