Some forest vipers can be found south of the Clay Jotun camp (25k, 50k Vanern Swamp zone). They are day creatures; they despawn between 09:00 p.m. and 05:30 a.m. During the night, you can keep yourself busy by heading north to kill at the clay jotun camp. Their repsawn rate is medium, and their drop rate is low (averaging one drop for every ten killed).
Level 44 turned-in 20 forest viper venoms for 132,000,000 experience each.