Enc/Mag/Nec/Wiz can create it true, but are not the only classes able to create it. Bards are able to create it in a "Song Writing Kit." There are different kits that can make it for different classes. It is not restricted to the "Spell Research Kit." Make sure you buy the right "Kit."
Also, the drop rate is about 30% for Raw Fine Supple Runic Hides at Goru'kar Mesa, hunt the "A Centar Mundungu," "A Centar Tracker," & "A Centar Shaman" to the north on the east side of the river.