I would guess the most economical source for coal would have been that which was unloaded dockside from colliers. Many of the eastern coal roads had marine fleets to haul coal to the New England ports from the North Jersey coal docks, Port Richmond in Philadelphia, Baltimore and Norfolk. I think the idea was getting the revenue for the maximum amount of the haul, rather than share with other rail carriers. NMRA Bulletin did a neat series of articles on some of these operations quite a few years ago. Some of the crafts commonly used were "Schooner Barges" towed by steam tugs. These ungainly craft had masts to carry sails and tillers to be operated by an onboard crewman.
They would be strung out on long tow lines behind the tug, 3 or 4 in a tow. Back in these "good old days", if the weather turned sour and the tug got into trouble, the lines would be cut to save the tug and crew. Those poor fellows on the awkward, heavily loaded barges would be left to fend for themselves. Didn't normally work out to good!
OK, now that I've wandered all over the east coast, at great loss of life (and probably readers interest), I think the W.W.&F. probably bought their coal from ships. They had the facility and I'm sure the price was right.
One question: Would a ship (or barge) normally unload it's entire cargo at a port like Wiscasset, or would it parcel it out at various destinations?