I use the reusable ones. I have some from many different stores that I shop at, but my Trader Joe ones are the ones I use the most.
Just FYI...if you have a Costco, they sell a 4 pack for about $5. There are two GIANT bags and two LARGE bags...and they are big!! I can put almost my entire Costco shopping in one or two of the bags! Ask about them there if you can't find them. They are really worth the purchase...even two packs if you do major bulk shopping! They can be used for so many things too. My son took some to hold all his things when he went paintball shooting with friends, he takes them to school when he has to bring a change of clothes for band, we take them to the beach with us to hold our snacks, towels, etc....
I just keep them in the car with me, and once you start bringing them each time, you won't forget. I like that Trader Joes has a sign out side their door that says there is still time to run back to your car and get your bags!!