Same CO2 cartridges that go in air rifles/BB guns. I'm not sure if they are the same but check out prices on Crossman and other air rifle companies cartridges.
Those set ups with the small cartridges have been around for a long time. When I was a kid my folks had a tap that used them with small kegs. They are still around and I still might have the tap somewhere.
Adding sugar and sealing the keg should carbonate your beer. I do it all of the time with 5 gallon kegs.