I just posted this on another thread. Have a read about how to store KM's hay in it's original box.
http://www.guineapigcages.com/forum/346055-post4.html.
You can also get large rubbermaid/sterlite containers and store hay in it as well but you have to leave the lid off. I have two large ones that I store hay in from the local feedstore. I normally buy burmuda by the bale and I can fit half a bale into one of my containers and the other half into the other container. I leave the lids off and the hay stays fresh (although the top of the 2nd bale will turn a bit brown from air exposure by the time I'm ready to use it).
I know that when I bought hay from KM, I had gotten 45lbs. of bluegrass and 55lbs. of 3rd cut timothy. That and half a bale I had from the feedstore lasted 7 pigs for a year.