Forbidden page

Salomon Raid Quest: Control Tower Barbatos will drop pages, in addition to gears, hearts, and homobabies.

However, it's a raid boss so you will be fighting other masters to farm the boss. Look towards the previous Onigashima boss gates to predict how early and often you need to farm.

https://fategrandorder.fandom.com/wiki/Raid_Quests:_Salomon

A couple reruns in the spring have pages. Have you done Edward teach's interlude yet? 10 guaranteed page drops.