Best place to find iOS Developers for part-time work

When using Upwork, I would always recommend that you invite candidates to apply to your listing, rather than just see who applies. You need to be proactive in finding the right candidate. Otherwise, you'll get all the people who are good at selling themselves, but not good at actually doing the work.

I've hired a bunch of engineers off of Upwork (Elance and Odesk before) over the years and have always had middling success. What's worked best personally was my personal network, which I cultivated from years of being a developer, going to meetups, friends with startups etc.

Hi, I am an iOS developer myself. I'm available for work. Let me know if I can be of any help :)

I can recommend you someone I can vet for. DM me for details. Cheers!