I started keeping mine in my car, too. I have 5, that cost .99, too. Bought them at the local natural foods store. My DH keeps 4-5 in his car, too, because we never know who/when, etc. will be shopping. He got his at K Mart and they cost 1.00.
Usually I go to Trader Joe's once a week to buy a few things I can't get elsewhere. I get one paper bag, which I use for my newspaper recycling.
I still use the plastic vegetable bags, though. Maybe I'll stop.