Check out Libreture's list of DRM-free bookstores [1]. I usually get my magazines and DRM-free ebooks from WeightlessBooks [2] and Smashwords [3].

Kobo and Google Play Books also offer DRM-free books, but you have to look at the book details (further down the book page) to see this information.

[1] https://libreture.com/bookshops/

[2] https://weightlessbooks.com/

[3] https://www.smashwords.com/