It's usually Bing, AFAIK they are the only one that offer a whitelabel/API solution. There were a bunch of founders in the search category at startupschool 201

It's usually Bing, AFAIK they are the only one that offer a whitelabel/API solution. There were a bunch of founders in the search category at startupschool 2018 and 2019, those that I spoke to all used Bing as that was their only viable option according to them.