I also carry my canvas bags with me and sometimes forget them in the car!
In that case, if paper is not an alternative I will use the plastic as there are numerous recycling centers now, WalMart is one of them. As for the garbage bags I found an on line store called Green Home that sells them, they are a bit more expensive but to me worth it and I am also hoping that one day the bio degradable ones will become cheaper than the others!!
Try GreenHome.com for a large variety of bags. As with every thing, education is the key to the start of change. I am so impressed with everyones insight on this. Keep up the great work!