If the library is internal (written by the company), then the company should keep the source code somewhere, probably in a different DVCS / VCS repository.
In that case you just have to ask a colleague where the source code is kept.
If the library is external, you can only see the source code if the project is open-source, in which case you just need to search for the project.
If the library is internal but the company does not have the source code (due to technical disaster, loss of servers, etc.), then it tells your superior that you can not progress in the project without the source code of that library, and he will help you as power.
Libraries (dlls) do not have a copy of the source code. They are written in a different language, IL (Intermediate Language), which are levels below C # and levels above machine language.
There are some projects that try to convert IL into C # (JetBrains dotPeek), but the result is not 100% faithful, and the original code comments are lost. These tools are mostly used for academic purposes (learning behind-the-scenes), almost never for professional purposes, and certainly your boss / manager should not put you in a position where you need to use these tools to do your job.