Some griffon gliders can be found at the tops of a big valley (anywhere near 50k, 40k Gripklosa Mountains zone). They are spreadout and airborne, so one needs to do a ranged pull to get their attention. Their respawn rate is medium, and their drop rate is low (averaging one drop for every eight killed).
Level 42 turned-in 19 griffon glider heads for 93,000,000 experience each.