Little Go source code
Little Go is an open source project, which means that not only you do not pay for the app in the App Store, but also that you can get a copy of the program source code at no cost.
The project website http://www.herzbube.ch/littlego is where you can download the current version, and also older versions, of Little Go's source code. Please consult the README document within the downloaded package to find out more about how to build the program executable with Xcode.
Third party software source code
A copy of the original source code of each third party software package that Little Go depends on (Fuego, Boost), together with any patches required to build a particular version of Little Go, will always be available from the project website. This ensures that Little Go remains buildable even if a particular source code version is no longer available directly from the third party software website.
Apple's "iOS Developer Program"
Please be aware that in order to run your own version of Little Go on a real iOS device (not just in the simulator), let's say your own iPhone, you must be enrolled in Apple's "iOS Developer Program". At the time of writing, Apple charges an annual fee to participate in the program, as well as requiring you to agree to certain terms that govern the program.