There's not going to be a buy it for life option because your CO2 canisters are eventually going to need recertification. But this is close
I have a 10 lb CO2 tank that you can get from any home brew supply store. Get yourself some hosing and fittings from that same source. Then get yourself a carb cap that fits on the fittings and on a standard plastic drink bottle.
Total cost will be about $180. But you will be able to get you CO2 tank refilled until it fails a certification.
I bought all this stuff when I was making beer but now I have unlimited soda water without device or brand lockin.