Source Code Lair

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).

What types of apps are you talking about?
It is possible to do this for some javascript based web applications but it requires expertise.
In most cases you can't really just rip an app and take its source code. For any app with a compiled backend, that would require very complicated and time-consuming reverse engineering from experts.

You can view a web pages source code, download all the html and javascript it generates and reprogram its functionality which although less-complicated overall, is still time consuming and requires experts.

Your best bet is to go open source. Look for apps on the internet that are available to the public.
One place to start is http://github.com. It is a repository where developers store their code for others to copy, use, modify and re-distrubute.
open-source apps are apps where ripping others people apps you could say is 'allowed'. Though it is encouraged to keep them open-source and not re-distribute as proprietary software among most of the open source community.

if you are looking for a ready source code then look at in some marketplaces like :

chupamobile
codecanyon
sellmyapps
.
.
.
but if you are looking to extract source codes from a published apps, you need to have some knowledge about reverse engineering, even with that, you will face another problem, generate a keystore to upload the new code to the store,

I bought some from codecanyon they work good. As far as I remember, Unity marketplace has some too.