A cost of a designer is going to depend on that designers experience and work history. I would say on average a jr. designer is going to be in the range of $20-35 an hour, a designer is going to be $35-60 and an senior designer is going to be anywhere from $60-125. Those figures will change depending on your location. For example a sr. designer in NYC is going to charge more then a sr. designer in a smaller town. As to where to find them, you can look at portfolio sites such as BeHance or Dribble. On those sites you can look around for a style that you are interested in inquire about prices from the designer. I hope this helpful. Good luck with your app!