I am currently finishing my PhD thesis (in Mathematics) and trying to find a post doc position.
My PhD supervisor is a very nice person and is, of course, one of my referees. However, most places need 2-3, so I am not sure who else to ask.
I have a secondary supervisor but we only talked once really during a formal thing. So asking a referee who does not know much about the work or the personality is a bit... weird...
I had a decent relationship with my Master's supervisor, but unfortunately we have not corresponded once in the 3+ years after my masters, so me writing "hey, do you remember me from waaaay back then, would you please write a reference for me" seems a bit ... strange .. too.
I have published several papers already but I don't have any co-authors whom I could ask.
I have given one talk outside my university but I am not sure who to ask there. I don't actually know these people, my supervisor referred me to them and they were OK so I gave the talk.