If you are wondering how to find a good application developer, you have to know that it isn't easy. Try to find any reviews about the company or get feedback from clients.
I think that the main factors are feedbacks and customers support. I can recommend using https://shakuro.com/mobile-dev.html , I was developing my apps with their help. They have a lot of positive feedbacks though.
Actually, it's not that easy to find a good software development service provider. You should focus on clients' feedback. Recently I came across a great service, I highly recommend SteelKiwi - https://steelkiwi.com/