Where do you buy your eBooks or have eBook subscriptions?

https://standardebooks.org/

My local libraries (via overdrive and libby).

https://openlibrary.org/ for very obscure things, though it's mostly image scans rather than epubs.

DrivethruRPG, DTComics, Wargamevault, etc. Always check out the deal of the day. Support indie creators and their passionate need to provide you with amazing memories...

Humble Bundle

Bundle of Holding

Google Play Books is IMO the most consumer-friendly standard ebook service overall right now, but I admit I don't really give as much attention to DRM factors in this area. Reviews from customers are not super helpful there so check elsewhere if you are a review-optimizer. But the deals are honestly really amazing; search for "the" or some favorite topic, enable price drops, set max price to .99 and ask yourself how much you care about DRM in these cases :D (I do care myself but it's a nuanced sort of caring). Their comics bubble navigation (swipe up) is great too. They don't take the price-obscuring approach Amazon does to upsell Kindle Unlimited or promote authors who pay more, or however Kindle's obscurantist search/browse is supposed to "work".

A weird issue with Google Play Books seems to be caching-related though...I often have to force quit and reload the app so I can view the items I just added to my wishlist xD

Project Gutenberg always...I have a daily excerpts page which draws from almost 500 of my favorites there (profile)

Oh and various other publishers like Evil Hat, Green Ronin, Steve Jackson Games, Troll Lord, Goodman Games, etc. Some of them host mind-blowing deals on their sites occasionally. IMO if you are a publisher you should always do this, it's one of the best ways to leverage being a small publisher in a big world.

Speaking of which, get on email newsletters for ebook publishers, they'll often include links to every service they sell on. If you like Star Trek books then get on Simon & Schuster's email list, this is like the best time in history to be a fan of Star Trek books... it's like a constant fire sale. Great strategy IMO.

Technical books using https://learning.oreilly.com/home/. I have been a subscriber for more than 10 years, dicontinued for a few years and resumed about a year ago.

Also use Amazon Kindle, Google Play books if I can't find the book on Safari.

https://libgen.is or my public library

Without further ado, here is a huge list of ebooks resources: https://custom-writing.org/blog/free-books-online

Mostly Kobo for fiction, sometimes Amazon; publisher sites for academic/technical books. I use Epubor for removing DRM. I don't buy ebooks that are only available in proprietary formats that require using a special app to read (some academic textbook publishers do this).

I use Libby on iOS to loan books from my local library and then I send them to my Kindle.