Amazon.com : 8" US box center fin Safety Flex Soft replacement for longboard SUP Stand up Paddleboard River Surf Whitewater airSUP AIR7 Skeg with optional 'No-Tool' Fin Screw (Black (easy screw included!)) :
Amazon.com : DORSAL Pintail Surf Fins for SUP Longboard Surfboard Center Velzy Style Flex-Fin Black 9.75" Polycarbonate : Sports & Outdoors
Single Fin Types: Longboard Single Fin Guide - WAVE ARCADE
9.25" Flex Series Longboard Fin - White Clear - Shapers Surf Co
True Ames Farberow Flex longboard fin - available in 9", 9.5", 10".