Aside from having a look into what can be discovered on Ensō, our suggestion would be to reach out to other characters, who may have requested additional plants or brought them to Ensō from other locations! His Stone of Far Speech was recently updated with a notes application, so retaining the info once he's gained it wouldn't be hard to do. If you'd prefer to have a physical catalog, the library may have some resources available that he can utilize already, or they can be requested on the Item Request page.