During nighttime in game , Shadai pop's up under the tent at the oasis past the Chimera's. You can get 1 of your 81 SB's from him for 200ish tomes , don't remember exact amount.You can get another doing the Gracia instances. The missing one/ones come from the epic bosses.
You can also buy some 81 spellbooks (dual dagger mastery, turn to stone among others) at the Gracia base, with quest items you can get from doing Hall of Suffering and from mobs inside the Tiat instance. Others come from the reward boxes you get from doing Hall of Suffering, or from the raid boss you can summon at staccato nest.
It really depends which 81 book exactly you are looking for.