I've been using the MillerCoors Home Draft units that come with the 16 gram cartridges. The 8 gram cartridges are cheaper by the gram, so I've constructed a little converter that I made from a spent 16 gram cartridge.
http://www.mediafire.com/imageview.php?quickkey=pohwdi1ae10fqrr&thumb=5http://www.mediafire.com/imageview.php?quickkey=7nfsycmb1ris7z4&thumb=5http://www.mediafire.com/imageview.php?quickkey=8afhv18db3661vm&thumb=5
I use 14 pennies to move it up to the right size, and use the spent 16 gram cartridge with the ends strategically cut off to center the smaller cartridge. I have had problems with the MHD units where the smaller cartridge was not centered, or the smaller cartridge heads were not the right size. But I've got it down to a science now, and I always buy the 8 gram cartridges, but use them in the taps made for 16 gram cartridges.
If you attempt this, you probably will need to adjust how tightly you tighten the cartridge holder; if you buy the perfect replacement cartridges, you twist it in until hit hits the stop. But if you improvise, like I have, you stand the chance of twisting it in too far, and binding the CO2 regulator. The symptoms you would see are that the CO2 wasn't flowing into the bottle. If you loosen and you get CO2 in the bottle, stop. If you loosen and get CO2 hissing outside the bottle, tighten again! I went through a buck or two of CO2 before I got the feel for it, but now I have (from Red Rock Minnesota) a big order of 8 gram cartridges that will last me a very long time.
--Dale--
Oh, and I have not seen anything but 8 gram nitrogen. If you find 16 gram version, please post.