After the first few times I had apple remove a book from my library, and the only explanation they ever gave me was "sometimes books change, and when books change they are a new book," I just went back to DRM free. If I have to jump through hoops, and still can't keep the content I legally purchased, why would I legally purchase the content?
Honestly, I havent bought a digital book since. You can read between the lines there.
I don't actually believe there will be a place to purchase DRM free books. Most of the ones I am seeing from a cursory search are for copyright free books.
kobo.com sells books without DRM (under eBook details they tell you the exact format). For example, the murderbot chronicles are sold without. Sadly they don't have everything like amazon does, especially for niche authors that only publish digitally.