1. Get your Ph.D advisor's letter first.
1. Ask others if they like to help. If so, then give them more information, including your Ph.D advisor's recommendation letter for reference. Then they will know you better and easy for them to write.
2. Diversity of letters. Letters from academic, government, even company, if possible. The higher rank for referees, the better. (e.g. you can try Chairman of your scientific society you belong to.....).
Just try, you never know. You don't expect everyone can write for you. But if you don't try, you got none.
Good luck.