From Fisherman's Haven, head north along the east side of the beach where there are two groups of Bog Skale. Turn west at the top of the area to the Tears of the Fallen portal near which there is a group of Bog Skale. Apart from a few drakes, all the creatures in Tears of the Fallen drop Bog Skale Fins. An efficient route is to go up the east beach where there are four skale groups to the north edge of the area, and then to go west and make a counterclockwise circuit of the lakes area. Rezone through the portal to Twin Serpent Lakes. With a single farmer, the drop rate is 4-8 Bog Skale Fins per run.
Exit Riverside Province into Twin Serpent Lakes. Just outside the exit is a group of Bog Skale Icetooth and Mergoyle. This is a faster area to farm, but slightly more dangerous. It makes up for this by allowing a group of 6 to zone in and out quickly killing and then zoning.