1) LinkedIn - Reach to 2nd degree connections, alumni etc.
2) HR - Speak to HR at the offices you're interested in
Some general rules:
1) Never ask for a referral upfront - it's rude. If they were interested, they might not be anymore. If they weren't, well now they definitely aren't. You let them offer, and you let them know you're interested in applying. That's it
2) Play tag - If you can hopscotch across people, that's the best way to land on a good conenction (don't be annoying about it though). What I mean is, if an alumni at x office agrees to speak to him, but you're fundamentally interested y office, let him or her know at the end of the call, and ask if they know anyone you could speak to to better understand that office. Same goes for other industries!
Some great Q&As already here: