Have you checked out this site?
http://www.jamesmccusker.com/shopping/Prices are high, but they do identify a number of the various cachets as outlined in the Mellone Catalog. Of particular note is that several examples may have the same Mellone Catalog ID Number, but are known in different colors, which creates yet another sub-variety. There are also the occasional "unlisted" cachets that aren't included in the Mellone catalog.
It can be very challenging to collect them "all".