1. Checking the Lessons tab in their online courses for a "Faculty Guide." Sometimes the Faculty Guide includes supplemental teaching materials (both those related to the textbook and not).
2. Finding supplementary resources actually published with the textbook.
Make a request to the publisher of the textbook or eTextbook.
Let them know you are using the text in a class with students.
They may or may not provide you with:
Physical materials
Supplemental materials
A login and password to an online site.
Note: eTextbook providers like VitalSource usually provide only the student copy of the eTextbook, sometimes, however, there may be links to additional materials. In VitalSource, check the My Bookshelf page for your eTextbooks to see if there are companion websites you can link to/ask for access.