The best way to discover new keywords for your Amazon Ads campaign is to run an automatic campaign, along with a manual campaign, with the same products in each. Once the automatic campaign gains traction, you can start analyzing it’s search term report to find good performing keywords and move them to your manual campaign.
Thus over period of time, you will accumulate all the good performing keywords in your manual campaign where you can adjust each keyword bid individually.
Karooya’s ‘Great Converting Keywords Report’ can help you do this exercise, easily and efficiently!