Co2 cartridges can be purchased online easily. Just make sure that they have the screw thread. A lot of online retailers sell them in packs, however you can purchase them from bike shops as they are used for inflating tyres and can be purchased one at a time. When you discharge the canister, you will lose the green 'arming clip'. The clip is simply a way of knowing that the cartridge is loaded and ready to go, and it doesn't actually serve a purpose other than that.
It is also useful to know that if you ever get in a situation where you need to use your PFD and you have not got an active gas canister, then you can always manually inflate with your breath into the tube on the jacket with two or three small breaths. Hopefully you never need to use the PFD in real life, but great to know you have it, just in case.