Whom to suggest to publisher for endorsement of my book?
As publishing has also a commercial side, publishers do want to get their costs paid (and they hope for some profit). So, they want to ensure as good as possible that someone (libraries, institutes, ..) will buy the book. For that you need your endorsements.
There is no problem in taking you supervisors for endorsement. However, I would advise you to personally ask them for their support.
Furthermore, it would be helpful if you find some other people that may to the same - have you been to conferences, spoke with people - do you built up a personal network with other peers? If so, ask them. They should be well known people in the subject you are publishing, of course. Reputation sells.