Bookshop.org lets you filter by drm when you search.
https://www.bloomsbury.com has drm free stuff, and after https://news.ycombinator.com/item?id=42811332 i respect them even more.
Also might be worth scanning old mentions of such https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
The bookshop.org filter works nicely. After searching for a keyword, pick "Formats/Ebook/DRM-free". I'll add a link and a note.
libro.fm for DRM free audio books, buying from there supports local book shops.