I have a reusable bag full of reusable bags.

Always decided to go to the supermarket off the cuff and did not have any bags so paid for more reusable bags.

Once it got to the point of having about 40 of said bags I has now resorted bag to using normal plastic bags. Which I collect up and then bring back to the supermarket for recycling.

I do also have a shoulder bag with lots of space that I use and try to carry a couple of spare bags around in it, but once they are used I often forget to put them back in.