I didn't have one when I first started - I was (and still am) at a startup as the lone dev besides the CTO. It was pretty brutal, to be honest. About a year in, we hired some more experienced/senior devs, and one of them has sorta taken me under his wings. Didn't even ask him to. I'm starting to realize that if you make it obvious and public that you're:
- keen on learning
- you're humble
- you'll run away with any feedback you're given
someone will often notice and want to mentor you. But if this doesn't happen, another thing you can do is find someone at a meetup and ask if they'd like to get together and talk about their experience.