There are many ways. None of them involve a short-time commitment. They all involve building a loose relationship over time.
Approach 0)
Spam everyone on LinkedIn. Bad.
Approach 1)
Go to a Hackathon. Maybe even participate (marketing/consumer research/design/code - if you can). Sponsor really good pizza OR really good coffee (not crap DD). Talk to developers. Find out what kinds of apps they like to work on. Follow-up.
Approach 2)
Throw your own mini conference. Can be anything from a panel. Make it valuable to the attendees. Ask for emails but don't spam the attendees. Ask them what kind of gigs/jobs they are looking for - tell them you'll keep an eye out.
Approach 3)
Since your friend works at a recruiting firm, have them hold a developer-only mixer, bring in someone who can give a bunch of tech talks. Give away a door prize of an iPad or a Raspberry Pi.
Approach 4)
Lots of developers would like to encourage younger developers. But they don't know how. Take the lead and sponsor and organize a hackathon with a local school (just one). Background checks aside (they may be necessary - for working with minors), the recruiting firm gets good press, the developers get good press, non-direct coding resume bullets.
Approach 5)
Build an app for a deserving non-profit who has a mission you believe in with college CS/design students who may want to get an app on their resume. Building the right team may be difficult.