The Cameron County (PA) Historical Society posted a link to online map of Emporium, PA on their Facebook page. It is part of the Sanborn Fire Insurance map series at Penn State's Digital Map Drawer.
So, first, I was excited to discover my Gaskills were apparently partners with the Whittemores in a lumber mill there. It must have been my great-grandfather, Everett, who moved to Emporium from Tioga County NY at the same time as family friend, Norman Whittemore. I'd never known about the lumber mill, so that was pretty interesting.
Second, a commenter posted that the Sanborn maps exist for most towns and cities in the US and Canada. So I found the Wiki entry and in the external links section is a listing by state. Can't wait to explore these.