What is the best way to farm combat manual I?

There are some great answers here that will directly help with your question:

https://www.appgamer.com/puzzles-survival/answers/35693-cannot-evolve-characters-anymore-because-of-insufficient

But basically there is no single best place to farm combat manuals. Keep going through the missions until you get stuck, then replay the missions you have already completed for more combat manuals.