I cannot evolve characters anymore because of insufficient material from Combat Manual I. I have looked..

Combat manuals are earned during the campaign levels. You can go back and play the levels again to earn more. The drops are random but you can tap on any campaign level to see what items it may drop as rewards.

Go to the last board you won but instead of competing...blitz

No best spot, you can preview possible rewards, my opinion is to do new higher maps because they give bonus loot first time. When you are beaten, cannot proceed higher then choose an easy level where auto fight is enough with your current heroes.

You have to obtain 30 combat manual 1. When you have enough the evolve button on the character you chose will light up and flash. Letting you know you have achieved enough. The manuals are in campaigns. Easiest way is go to level 1 on campaign. And see what items will drop on each course and go to blitz. Remember, you have to get 30.