Files for Boards added with the board manager end up in weird places, dependent on OS and such.
The easiest way to find them is probably to turn on "verbose" compilation and copy the output somewhere where you can read it easily, and copy from there. On windows, it'll probably be something like
\AppData\Local\Arduino15\packages\arduino*
(Note that "AppData" is normally a "Hidden" directory. Sigh.)
I like to use the "portable" feature, in which case things end up in /portable/packages/arduino/*
(which requires that you have write access to the install directory...)