Your advisers, journal editors that you've reviewed papered for, the people
you met and had discussion during a conference might be good options. If you
still feel it hard to find references, you can ask your adviser and see if
he/she can help you using his/her networks.
It might take you a while to contact and ask for the recommendation letters.
If you are aware that your references are going to be too busy to draft the
letter for you, I think you can hire your own attorneys to work on the
drafts for you and then you can send those letters to your references to
review, make changes and sign, which might be much more efficient.
