You don't have to get it. It is loaded into the system when you install the board manager package for the R4 board. So you should just be able to use it.
If you want to look at it then on my mac it is stored at:-
/Users/your_username_here/Library/Arduino15/packages/arduino/hardware/renesas_uno/1.0.2/libraries/AnalogWave/analogWave.cpp
Change 'your_username_here' to your user name, copy it, and then select "Goto Folder" from the Finder's 'Go' menu.
Not sure how to get it from a PC or Linux.
At the moment the documentation is being written. So far it is not very comprehensive and there is a lot about this library that is undocumented. You can learn how use it by following the tutorials, but there is a lot more to this library than is covered in the tutorials.
For the latest documentation see:-