This one’s easy enough. I’ve stashed a few (of my millions of) canvas bags in my car and refused plastic bags when I’ve forgotten my bags in the car.
Today I used one of these bags to carry my lunch in, which further gets the canvas into my consciousness and keeps me remembering it. 🙂 Last night I bought yarn and accepted no plastic bag for it, then stowed my purchase in the bag I knew was (finally) in my car. When I stopped at the grocery store a few minutes later, I took the bag inside with me and used it as a shopping basket as I browsed around. So far, so good!
And that’s just a single search of Instructables.com!
You can purchase a bag of course, but be aware that this will include packaging and implied waste/costs. If at all possible, avoid buying new things to be “green”. You definitely have your own resources at hand!