Of course you could use different freelance websites, try to find good developer on iOS/Android forums or collaborate with professional and well-known developers team.
By the way I have the successful experience of cooperation with this mobile development team: Rozdoum. Software development house.
These guys have excellent skills and pleasant prices. They build mobile apps for Android and iOS and work with such projects like social networks, conference mobile applications, personal applications for business and location-aware apps.
Here, you go with some very important points that you can utilize when looking for a reliable Mobile app developer or company!
1. Apart from all technical aspects, a developer must identify client’s exact requirements. It helps them to make an app that can really go fascinating to fulfill their user’s various demands.
2. It’s very important for developers to judge how actually the apps would look and work. This can remove future chances of bugs and other technical errors. It determines the fulfillment of client’s every need.
3. The UI of an app is an important aspect and the developer must make it very user friendly.
Logic and experience plays an important role in addition to one’s technical expertise. Make sure you choose the one with sufficient practical exposures!
Hope, this would help you to choose the right one.