A list of people who are willing to provide valuations (free or for a fee) can be posted. Each person's entry would contain a brief description of qualifications as well as contact information.

A group of very knowledgable enthusiasts. I assume asking for an estimate for "valuation" is likely OK since is it an antiquarian item, especially so because it is for institutional support of the Franklin Mining Museum, rather than simply personal gain.  You could ask the admin, Roger Lang directly if requesting a valuation estimate would be OK in this group.