I’m fairly certain you already know all the sources of Mementos, and you’ve just narrowed it down to sending Watchful letters as the most optimal solution. In the case that you didn’t, if possible, there are actually two more ways to grind.
The first option is to send letters. Yes, obviously you’re doing that now, but you could also send additional letters in Gant or coded instruction if you know quite a few Licentiate friends or are a Midnighter yourself.
The second is to grind in the Chessboard in Parabola. The two mates and the draw offer all give a Memento on success. Obviously, this option is pretty bad, since you can just ask everyone on the forums anyhow.
With due permission, I would like to provide my humble aid towards the same goal: if anyone is in need of grisly case notes, do not hesitate to ask me.