Grab one of these from any major food or department store, Matt. One of the handiest things I own for both kitchen use and for frozen rodent storage. I got mine on sale for only $3.00 but generally they run around $8.00 I think. The bags aren't horribly expensive either. It works like a dream!
Off topic, but last spring I was walking out of a T station in downtown Boston on my way to work and people were handing these out for free. I got two of them but I saw people with like four or five. They actually work pretty well.