I would be surprised if the Rockwood marina didn’t have fuel as large as it is, boat dealer, wardens keep their boat there, etc. Website doesn’t mention it, maybe worth a call.
Gray ghost Camps on the river has fuel, but may just be for guests? Amy and Steve are pretty responsive, maybe shoot them an email.
Birches has a fuel tank in their parking lot off the dock. Worst case you could use a 5 gallon jug.