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.