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.
They have bags to freeze milk.
Put in the fridge to thaw. Or over warm water, if you have to.