+this. You can buy them in qty ~50 or so on Amazon fairly cheaply. There are probably a billion, but we have a big stack of these we use for everything from prep to freezing leftovers/taking to work: http://goo.gl/viX1Nh (they come in 8oz versions too, which I also have a stack of). They crack after about 10 cycles through the freezer and in backpacks/lunches, which comes out to about 5c per use. Very reasonable.