Bravo! I wish more people were using canvas bags - I think they're great, and if more folks use them, maybe the baggers will stop giving me funny looks. =)
If you ever go to Lakewinds or Valley Natural Foods, they sell them there.
If you or any friends ever go to conferences for your jobs, you might want to check out the giveaways at the booths - a lot of them give out bags because the attendees are loaded down with papers, etc.
It's over for this year, but the Living Green Expo is also a great place to find good bags (and lots of other good stuff): http://www.livinggreen.org/ (admission is free).
There are three things I'd recommend checking out before you buy or take a bag. They'll make a big difference in how much you like shopping with a canvas bag:
1) The mouth of the bag should be as wide as possible - ideally, the bag should be wider at the top than the bottom.
2) The bag should be at least 2'x2' The small bags just won't hold enough. Your bag should be able to hold at least two full size cereal boxes.
3) Try slinging the bag over your shoulder - the straps on the bag should be big enough that it doesn't pinch when you carry it this way.
As an added idea, there's a great local company that makes fabulous drawstring bags you can use for produce. I really love them, and I get lots of compliments on them when I use them to shop. They're called BYO Bags and they're by the local company Cool Hats: http://www.coolhats.biz/byobags/BYObags.html