Consider searching .edu sites for syllabi – every syllabus I have ever received had the required textbooks on it.
You could also try searching for ISBN’s on university domains, but I imagine this will be less consistent.
My university partnered with Barnes and Noble and published a list of required books per course as a bundle option. If a course catalog is available you can further refine your search based on course numbers (like FIN 4401 or CSE 4096, etc). Look for any partnerships a university has with a bookseller.
Those are some thoughts… good luck!
With the increased use of closed Web portals, whether bespoke (or more likely, OEM rebranded), or third-party, access to basic course information is increasingly difficult, often hitting a password-protected registration wall.
Emailing faculty or departments directly may offer (occasional) success, as can tracking down reader lists at campus bookstore websites.
There seems to have been a brief golden decade of roughly 2002–2012 when handcrafted professor homepages included course lists and syllabi at least at major universities, both public and private. It’s grown markedly worse in recent years.
Open Courseware, under that name frequently, is an exception. M.I.T., Harvard, Stanford, and Cal Berkeley, off the top of my head. CUNY and Columbia likely as well.