Dear Mr. Kelly,
Thanks for posting your request on History Hub!
We suggest that you review the following sources from the Massachusetts Historical Society for possible leads on the stereoscopic photographs you are seeking.
Since you mentioned you have been checking eBay, you may be interested in visiting the Brimfield Antique Flea Market, a very large, open-air flea market in Sturbridge, Massachusetts that takes place 3 times yearly. There are many dealers of antique ephemera and photographs who may be able to help you.
We hope this is helpful. Best of luck in your search!