I use this. It's very compact and it takes threaded or non-threaded 12 g cartridges and all sizes of the threaded variety. The pumping option takes some practice, which, unfortunately, I didn't do before I need to fix a flat.

http://www.performancebike.com/shop/...tegory_ID=4362

I guess what I don't understand is the need to get to 120 psi. In most cases, isn't 80 psi good enough to get home or back to your car? In other words, isn't the 12 g cartridges sufficient?