I would think that rather than focusing on those clubs that have the most material to offer, you would focus on clubs with the most significance and then set about collecting material, regardless of whether such club's currently possess the most material. As this website evidences, material can be found by the intrepid.
Which clubs are the most significant is certainly debateable, but certainly few would argue that to the USGA, the logical answer is its five founding members, Brookline, Shinnecock, Newport, Chicago and St. Andrew's.
No?
I've been trying to follow the progress of this with great interest; to the extent you can, please keep us updated. Good luck with the project.