Boost manuals are required to enhance weapons and armor. The shops provide only minimal equipment, so you must obtain the boost manuals if you hope to survive. There is only one way to get the manuals: you must find a mysterious man named Kurius who will give them to you.
Kurius is present in almost every area of the game (both towns and dungeons) and will give you a few manuals every time he appears.
It is critical that you do not miss him and skip any manuals, because if you do, it is likely your enhancements will stop short of the maximum.
The best way to avoid skipping manuals is to check the "collections" from the title screen every time you obtain manuals from Kurius. If you see a "hole" in the collections list, you probably missed him in a prior location, so backtrack and look for him.
His final appearance is near the beginning of the last dungeon, Galbure's Tower. If you don't see him there, that means you missed some manuals along the way (you will definitely see a gap in the collections list).