Struggling to find endorsements for my polished manuscript. How did you get endorsements?

Hi everyone,
Are there websites or ways to contact people with PhDs (in subjects relevant to my m/s) who would read and endorse for free or for a fee?
I dont have impressive credentials so I need quality endorsements from people who are experts in subjects such as history, politics, economics.
So far everyone I have emailed is too busy to read a whole m/s.
Maybe I should ask them to read and endorse 1 or 2 chapters (each chapter stands alone)? and collect endorsements that way?
I am grateful for any ideas. Thanks.