They have bags to freeze milk.
Put in the fridge to thaw. Or over warm water, if you have to.
I used the bags from walmart, Lansinol i think. Label em and lay them flat in the freezer. When they are solid you can stand them up in a rubbermaid box to save space. When you need to use one, just fill a coffee mug or glass with hot tap water and let it sit in there for a few minutes. Do not miceowave it because it loses nutrients and can create hot spots.