There are quite a few places especially technical ones. However, many major pulishers don't offer them, so you then have the choice of DRM stripping, searching for illegitimate copies or giving up/in and using a DRM copy somehow.
https://www.ebooks.com/en-nl/drm-free/
https://www.libreture.com/bookshops/
Note places like ebooks.com don't sell exclusively DRM free, so you will need to check per book. I returned one to them and got a refund because of my mistake in buying a DRMd copy.
> Standard Ebooks is a volunteer-driven project that produces new editions of public domain ebooks that are lovingly formatted, open source, free of copyright restrictions, and free of cost.
(Not affiliated, just think they do great work!
Discussed before:
They somehow watermark your books though, but they don't contain any DRM.
O'Reilly was very good. Now they only rent their books via Safari. I feel betrayed.
One could argue it is ethical, if not legal, to obtain a drm-free digital backup from a pirate site if one has a physical copy on their shelf.
Only for certain books.
Book DRM has been a nightmare. Tying a book to certain readers is an automatic showstopper for a lot of folks (like me). I recently bounced a tech book I purchased, because I had to download their crappy hybrid app to read the book. The same people that sold it to me, used to sell unlocked reflowable EPUBs (my preferred format), or watermarked PDFs (I have no problem with simple margin watermarks). I'll lay odds that the app they make you use phones home. I didn't bother downloading it, or reviewing their privacy policy.
I'm not so concerned with privacy, as I am, with perpetual access to the books I want to read, on whatever reader I want to use.
I often go back and re-read books, years after the the last time I read them. I want to have that book available, then.
Also, this goes for all types of media, region locks are garbage.