I got one ruined batch once. I've since gotten a set of cheap airs lock from here: http://www.amazon.com/Piece-Plastic-Airlock-Sold-sets/dp/B000E60G2W/
I then punched a small hole in the lid of a wide mouth jar (had ghee in it before, all metal lid, so no BPA lining) and used gum to seal it.
I also found that using lots of salt between each layer of sliced cabbage, pack the jar all the way to the top, and then adding some water at the end, maybe half way up the jar helps kick start the fermentation.
Last batch, I also threw in some beets.
Be sure to fill water to the fill line of the air-lock as this is what keeps air out. As the bacteria ferment and produce CO2 and other gases, the gas bubbles up through the air lock water and escapes, keeping the internal pressure the same.
Doing it this way doesn't require a special expensive ceramic pot, nor weights, and is dirt cheap.