The source template files are found in Open-ILS/src/templates/opac.

These template files are installed in /openils/var/templates/opac.

NOTE. You should generally avoid touching the installed default template files, unless you are contributing changes that you want Evergreen to adopt as a new default. Even then, while you are developing your changes, consider using template overrides rather than touching the installed templates until you are ready to commit the changes to a branch. See below for information on template overrides.