I have a Kobalt CO2 regulator and hose kit from Lowe's. Uses paintball-sized canisters. Lowe's used to sell them and you could exchange the CO2 tanks there, but they stopped selling the regulators and the tank refills so now I go to Dick's sporting goods.
The tanks are certified for maybe four years during which time you can get them refilled. After that, you have to buy replacements. I think the tanks are maybe $12 and refills are under $10.
The tank and regulator hang off your belt and it's great for small jobs. Works really well with a pin nailer or finish gun. Would not work with a framing nailer. I use it in the shop when I have a small job and don't want the noise of a compressor. It's really nice for a quick trim job. Had some base shoe on a kitchen remodel after the painting was done and it was nice not to drag out the compressor for maybe 40 finish nails.
Amazon and others still sell units. I'd suggest using with the coiled short 1/4" hoses to minimize the amount of air that fills the hose and conserve it for driving nails.