Anna's Archive and Library Genesis are good places.
Piracy
Welcome to /c/piracy
No netflix or streaming services landlubbers allowed, this is pirates territory.
Site: libgen[dot]is for probably 99% of all books with a pdf that exists online. However this site often gets blackholed by most institution's DNS/firewalls because they don't want you using it obviously. Can bypass using a VPN or TORbrowser.
Generally when I do any pirate stuff via browser only I just use TORbrowser. Yes it's not endpoint secure but it just needs to be enough so my ISP doesn't complain. The safest option is always to purchase a trusted VPN which will usually get you through any institutional firewall as it's an "insulated pipe" that encrypts all traffic inside it.
Yes, torbrowser should get you around it and i know libgen has a tor mirror download link on books
If you don’t want to use tor and still want to download (tor can be slow) you can also change your dns server if your wifi blocks it through dns. No need for a vpn
Get a library car and casually go take photos of each page.
Lol, implying that universities nowadays actually have their required textbooks in the library. Maybe a couple 101 courses (enough for 1 in 200 students) but they have a direct financial incentive to not offer course books in the library.
You can also check Z-library. If you can't find what you are looking for and you know that an ebook version exists I may be able to get a copy of it, feel free to mail me (see address and GPG key on my profile).
Lots of good advice in this thread, BUT I would set expectations with your family that you probably won't be able to find brand new textbooks. Books generally don't get uploaded unless it has been a few years since they was published. So check the publication date before you get your hopes up.
Publication date doesn't really matter much at my uni, since the only differences would be chapter and page number. I easily make do with the search function in the pdf. ...for now. I'm not sure about upper level courses yet.