xcode development

Does anyone know where to find source codes to existing apps? I want to edit them, run them on my computer, and then sell them. Kind of two questions in one, because I'm not sure exactly where I should/could run the source code (I'm beginner).

Hi, I hope this is posted to the correct areas. Need a developer that can copy some simple apps in app store. Might add a few small features. I can do the basic graphics, I need you to do the back of house stuff. Nothing crazy. This will be the first of many apps and updates for the right...