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.

Anna's Archive and Library Genesis are good places.

Get a good VPN Proton or Mullvad if possible. My goto website for text book is Library Genesis. You can download epub, or pdf of particular textbook..

Libgen

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.