The Glue code for those textures haven't changed since MoP at the latest. The online repositories containing the Glue system that I'm aware of only go back that far. The classic clients use the selection texture for both selection and highlight. It's actually one texture object defined as a highlight and the show state is locked when selected.
The internal path for it is
Interface\Glues\CharacterSelect\Glue-CharacterSelect-Highlight .
There aren't any repositories for the actual texture files themselves that I'm aware of, but you can
extract them from the current clients with the
ExportEnterfaceFiles art console command (only available from the glue screens and not the in-game session).