If you can possiby afford it, I'd sugest you get at least a 5lbs. tank. A 12oz. tank is probably a paintball tank. Not only will it require you to get special adapters for it to fit on the regulator, but unless you have a shut off valve installed, you'll lose some CO2 when you screw it on.
The initial cost of a CO2 tank or replacement plan may seem high at first, but once you've put down that 80 bucks or so, it's cheap city from that point on. You're only going to pay 10-15 dollars for a refill, and they will be a very long time in between. I've been running a 5lb tank on my 65 gal since may and it still feels like it is over halfway full with the highly accurate, percision "pick it up and see if it's heavier than an empty tank" test.