Where would one find the source code for unrealpak?
Its included when downloading the sourcecode for the engine. When opening the visual studio solution, one of the programs is unrealpak.
Or you can simply look into the github repo: https://github.com/EpicGames/UnrealEngine/tree/release/Engine/Source/Programs/UnrealPak
(Note you need a connected github account: Unreal Engine 4 on GitHub - Unreal Engine)