Hiring a dedicated iOS app developer from India is quite simple. Search on Google using relevant keywords or go through agencies like Clutch and look out for agencies. Review the web presence which means their website, blog, client reviews, dribble profile etc to get an idea of the kind of quality, quantity and experience level they have and contact them to schedule a direct meeting.
There are two main focus areas to make your outsourcing experience successful. i) Communication and ii) Quality. Do ensure these two areas are seamless from your chosen service provider to double ensure success of the project.