From a Leggett, CA to San Diego you'll be able to find a canister every 150 miles or less, most coastal towns have some sort of sporting goods or camping store. I haven't ridden from the CA border to Leggett. I rode in WA and OR as well a few years ago, and my impression is there are plenty of potential sources there as well, but we had a white gas stove that trip, so I'm not 100% sure. Can't tell you anything about north of the border specifically, but I did travel in the Canadian Rockies and across BC with a butane stove and had no trouble sourcing canisters.
I generally carry 2 and start looking to buy when I'm down to 1.
Hope that helps at all - obviously leaves the question open for the more remote sections of your trip.