In larger "regular" supermarkets, I usually find it in the Asian or International Foods aisle. It's in a can, about 14 oz., I think. Usually there is coconut milk and coconut cream - the cream is thicker and fattier.
I've never tried to keep it once the can is open. Most of my recipes seem to use up the entire can. I think you could probably transfer leftovers to a jar with a tightly sealing lid, and keep it for a few days. I'm not sure though.