I'm not completely sure about the C02, but we have oxygen/acetylene tanks in our garage for welding that have been there for a long time, at least a few years, (as we rarely do any welding ourselves anymore) and they still work just fine when we do occasionally use them. I know the cylinders/tanks are dated, as they have to be pressure tested every few years, but if you swapped it out for a new one every other year or so I don't think it would be a problem.

You can even get ~20oz CO2 tanks for paintball guns at a few stores now (lowes, walmart, ect). Not sure how much dry ice they would make, but they would probably be a lot easier to handle than a big tank (which would either need a siphon tube or be turned upside down to get the liquid), as well as probably being easier and safer to store. Then all you would need is an on/off paintball gun ASA (~$20) and a length of 1/8" npt steel-braided hose ($ depends on length). You just hold the bottle with a gloved hand, turn it valve-side down, and open the ASA for the liquid CO2 to come out the hose.