Find them locally, and buy them a beer.

The best way to get developers to complete a customer interview is to go in asking for advice. Position the interview as some research that you're doing for a blog post or paper - that way the feedback isn't filtered based on it being your idea.

As for finding mobile app developers, the good news is that they're listed in every appstore on the internet. Maybe start with new & less noteable apps so that you get a good response rate from an email to the founder (usually the developer).

The other approach I've done is run ads on Facebook to a survey, or work with a company like SurveyMonkey.com to get answers (although I'm not sure if they can target mobile developers).

You can always use Clarity to reach out and conduct interviews - I do this often. Here's a search result for iOS developers https://clarity.fm/search/developer%20ios

If you're able to conduct your interview virtually, you could post on reddit or hacker news. Other ways include attending developer events and asking the developers in person. Depending on your area, you should be able to find developer events on meetup.com.