I never find a correct sized coin when it's really needed mostly when I go for shopping.
You can buy a premade key ring which has the correct size and can replace the coin.
But I found a very good tip to have the ability to unlock a shopping cart with something all of us have at hand!
Step 1: Watch the Video
Get a rounded key and use it like a coin ;)
There is no exact steps the video include everything you need to know about how to unlock a shopping cart.
It can be so useful so check it out!
Thanks for your support!