7th Singularity Babylonia: Observatory (Mother Sea) free quest is the best AP & time-saving location to farm Seashell of Reminiscence. You'll get about 4~5x Seashell of Reminiscence by spending 210AP.
3rd Singularity Okeanos: Archipelago (Hidden Island) free quest is the first location to unlock where you can farm Seashell of Reminiscence.
You can also farm Seashell of Reminiscence Sunday Daily Quest: Saber Traning Ground. For other item drops, check the table below.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
Saint Quartz Farming Guide
Nero Fest 2017 Autumn Exhibition Match (2016 ReRun)
How to build an Arts team
Altria Pendragon (Lancer)
Cu Chulainn (Alter, Berserker)