Can't find the opener? You could open the bottle with a countertop or even a newspaper, but Instructables user mikeasaurus has a good idea to avoid future hassle: just attach the opener to the bottom of a coaster.
The full Instructable might not be for everyone, where mikeasaurus builds a new set of coasters with built-in openers. It's a lot of woodwork and craftsmanship, but if that's your thing, then it could be a neat weekend project.
If you don't want to go that effort, a simpler option would be to stick a simple metal opener to the bottom of your coaster, with the opener's end sticking out. To make the coaster balance well, you could use Sugru at the four corners so that the opener doesn't make the coaster wobble.
Coaster Bottle Opener [Instructables]