These are notoriously expensive and hard to find. Although you could buy them individually the most desireable is the set of six in the presentation case. The case is green velvet and has a white cardboard slip cover as seen here. Very often the set is missing the slip case which naturally devalues the set.
Many, many times rarer is the set of concept coins. These were rejected and the heads were made bigger in the final run that was put on sale. You can see a proof set here under the merchandise set for comparison.
Happy collecting!