....on an ice stamp? How do you use them and do they work? Where is a good place to get one?

Not sure which store it came from (likely Etsy type), but it's my initial on it, about 1 1/2" in diameter.

Best way to use it so far has been to heat up a cast iron skillet with the stamp on it, and stamp away. I have done them in the glass and in the silicon ice trays when trying to get them ready for a party/gathering. It works better individually in the glass after the drink has been poured. I usually only get 3-4 stamps in before it needs to be re-heated. I'd advise a wooden handle to make it easier to touch in the event it gets too hot from your skillet.

A neat trick you can do is drop some dark simple syrup, angostura bitters, or luxardo syrup on top to make the design in the stamp stand out.