I get mine from Columbus foods. They are rock hard chunks, break them up with a small hammer or cut off pieces with a sharp knife, then melt in a bain marie or nuke them.
Makes your soap very hard and if you get in non-deodorized adds a slight chocolaty scent. If you only want it for the properties, not the scent, make sure you get deodorized.
Cocoa butter is kind of spendy, but it's very stable. You can buy in bulk and not worry about using it quickly. It will stay good for up to 5 years.