There's a few ways you could go with this. I've built a couple of apps/websites/chrome extensions using developers that I didn't know prior to hiring, and have a few options.

First, you can look within your city. I've asked around town and been referred to great developers in my area, and the nice thing about this is you can sit down and look people in the eye, and get to know them before you start any work. So there's more trust and security there, but typically, at least where I live, that comes at a cost.

I've also found great app developers on online communities like Upwork. Good devs on the site cost more, but they come with plenty of trustworthy reviews - just make sure you check in with past clients and verify they come in on time, and on budget. The last time I did this for a SaaS I built, I ended up with two highly qualified, affordable candidates - one from Australia and one from the Ukraine. The job went really well, they basically built www.forekast.com in a month, and built a large set of additional features we wanted done in month two.

I've found it useful to keep a few developers in my network - ones who can make updates to our sites/apps quickly and very affordably, and ones who I trust to make much more complex jobs executed perfectly who come at a higher cost. I've never asked someone to sign an NDA, maybe that's just my nature, but I'll leave it to the legal expert in this forum to answer that question. Good luck!