I'm on gazebo11 and I found libStaticMapPlugin.so in /usr/lib/x86_64-linux-gnu/gazebo-11/plugins. The plugin also exists in the main repo for gazebo-9 (https://github.com/osrf/gazebo/tree/gazebo9/plugins), so I think it should be similar in gazebo-9.
Did you source the setup file (/usr/share/gazebo-9/setup.sh) before running? If the output of echo \$GAZEBO_PLUGIN_PATH doesn't contain the path to the so file, the plugin cannot be found.