I recently bought a couple of these nice silicone molds for making ice balls, and they should be available in your local MUJI store or online. Unfortunately, they're quite expensive, which means you have to spend a lot of money if you want to be able to make many ice balls at the same time. You pour the water into a hole on top of the mold, but since water expands in the freezer, you shouldn't fill it completely... I tried a couple of times before I found the right amount. When it's completely frozen, you open it up, and voilà! Just be careful so your fingers don't get stuck to the ice.
In conclusion, a nice addition to any bar despite the price and amount of work.