Company doesn't assign mentors. How did you actually find one who'd invest in you?

Small startup, everyone's slammed, there's no formal mentorship. I know "find a mentor" is the standard advice but nobody tells you the mechanics when no one has time.

Did you just start asking a specific senior to pair regularly? Find someone outside your company? I don't want to be a burden but I feel like I'm leveling up slower without someone to bounce things off. What actually worked — inside or outside work?