Generally it is located in the first path in $PYTHONPATH, if this variable is set. If it is not set, it is located in path/to/current/python/lib/python2.7/dist-packages.
You can specify other install location via --root, --target or --user pip option. When using --user option, you should have set PYTHONUSERBASE system variable before.
If you are using system version of pip and python (installed through system packaging manager), then search in system paths. For Debian it is /usr/lib/python2.7/dist-packages.
There are no binaries like 'matplotlib', only library files are present.