I found it for sale in Lower Jeuno at the magic shop there. It is next to the goblin shop, there are 3 females behind the counter who sell the Scrolls, it was just over 14K.
Quelpia, the white magic seller in Selbina, only sells the single target versions, Protect II and Shell II. For Shellra II, I think you need to go to Waag-Deeg's magic shop in Lower Jeuno.