Glad to hear you're on the search. Let us know how it evolves. When I was in the area, I was able to circle the island in a boat with a local, who pointed out a trench like area where the quarry may have been. But now, the island is privately owned by two families from Montreal who use it as a vacation retreat. From the historical sources, the stones were shaped to size in and around Derby, Vermont, so that might be a good area to search. I passed through there briefly in returning from Lake Memphremagog, but really did not have much time to make serious inquiries.