Check with a sporting goods store, or a large sporting goods department at a discount store like Wal-Mart, for paintball supplies. Many of the CO2 inflating cartridges are also used for powering paintball guns, and you might be able to find the exact replacements for much less than $20. $20 for a box of them would seem more appropriate.

With ANY inflatable PFD you must be certain that there is absolutely no way that the thing can accidentally inflate and trap you in your boat. A fanny pak type of inflatable device, for example, could expand under the deck and prevent you from being able to exit. A vest type of inflatable device might inflate with a portion of the vest under the deck, and the remainder above the deck. If you have a small cockpit coaming it could seal you in place.

A non-inflatable PFD is much less money, needs much less care, and is always ready to go to work. They are available in many different styles, so it should be easy to find one that does not interfere with your paddling.

You might want to try making your own manually inflated vest -- very similar to making an inflatable float bag, but a different shape. Use an old PFD for a general pattern, cut the pieces from a rubberized, or vinyl-coated fabric and add an inflation tube.

SCUBA divers use similar inflatable vests, called Bouyancy Compensators, or BCs. These are fabric vests with inflatable plastic bladders inside. You might get a used/patched inner bladder cheaply at a dive store (usually they are discarded when a new one is installed) and play with that, too.

Use this in addition to an approved PFD, of course. And pratice with others nearby. If you find you like the idea of using an inflatable PFD after playing with these other versions then go for it.

Just my opinion(s)

PGJ