Here are three ways to get google custom search for free withoud ads.
- NON PROFITS – Register as a non profit with google from here. https://www.google.com/nonprofits/
- EDUCATION INSTITUTIONS– Option 2- You are an educational institution- Register for google for education from here https://edu.google.com/k-12-solutions/g-suite/?modal_active=none
- GOVERNMENT AGENCIES- Fill out this form to register as a government agency from here