A wishing stone is a found rock (pebble/stone) with an unbroken white line all around it. I have seen one, but the line did not go all the way around. So, sadly, it did not count. But, I made a wish anyway.
To make your wish trace your finger around the white line and think about what you want to wish for. Then, toss the stone into a body of water. You wish may come true.
You can also give them to someone else. But, apparently, they are not recyclable or returnable.
I have a feeling a bought wishing stone isn’t the best option. But, you can find them that way, if you want to try.