Many LHBS' have them.
I use them with my Coopers Kits.
Pretty easy, reduces steps to just dropping a couple in a Coopers PET bottle (24oz), filling and screwing on the cap real tight.
Just don't forget to add them! LOL I had to re-open a few on one of my first batches, add the drops (I forgot) and reseal the bottles.
Notes on that include you need to remove the little plastic ring that came off the old cap before screwing on a new cap (that has a ring). No, don't ask me how I know this.
The combination of the carbonation drops and the PET bottles make the bottling phase about as simple as you can get. Of course, racking to a bottling bucket (has a spigot that accepts a bottling wand) and adding bottling sugar is virtually as easy but Coopers system with their stuff makes it easy to just primary ferment and bottle from there after a couple weeks. A 2/2+ system that seems to work very well (for ME).