CO2 cartridges

Anybody know where to buy 12 or 16 gram threaded CO2 cartridges for cheap? This weekend in Baja I burned through 7 cartridges repairing a rear flat tire. I was real disappointed in their use, 2 12gram cartridges only brought the rear tire to 9psi. After this weekend, I wouldn't feel comfortable riding with fewer than 10 cartridges.
I'm hoping the larger 16 gram cartridges will work better, but all the cartridges seem way over priced.

Thanks for the info Wayne,
My buddy had a combo pump/co2 inflator with him. The manual pump was just about useless, the nozzle would fall off if you pumped to hard. It supposedly would take threaded or non threaded CO2 cartridges. The non-threaded cartridges may be the way to go, they seem much cheaper. I've found 16g and larger non-threaded cartridges for 1 buck each on Beer Brewing websites. The BB gun co2 cartridges would work but they are small, it would take a minimum of 3 cartridges to fill a tire.

At this point, I think I'm going to get a good small bike pump. I mainly ride in remote areas of Baja. After my last experience, I don't feel safe with CO2 only.

Jim