Located at the intersection of Route 175 and Route 25/3 in Holderness, across the road from Little Squam Lake,
https://www.facebook.com/NHmobilemarine/ seems like a good place to go looking for a 10-20 year old Mercury two stroke outboard.
Just up the road, also on Little Squam, there's Squam Lake Boat Livery, for used Evinrude and Johnson,
https://www.facebook.com/squamboatlivery/, and River Edge Marina,
www.riveredgemarina.com, for used Mercury and Honda.