For a larger initial investment you can get a co2 regulator, co2 tank and carbonator caps. The co2 will probably last you for years and quickly make up the difference in cost… and you can carbonate any liquid you want.

I found the easiest approach was to buy a little adapter and refill their proprietary bottles from a 5lb tank. It costs me like $30 to refill the big tank and I only need to do it a couple times a year. And that money goes to my local homebrew store, not genocide supporters.