Google Play Books allows you to download an EPUB, but only if the book is from a DRM‐free publisher. Tor Books is one such (sci‐fi) publisher, so I buy their books through Google Play.
I don’t know a way to check in advance if a Google Play book is DRM‐shackled. Hopefully, if a purchase does turn out to have DRM, Google would let you return the book for a refund if you haven’t read it, but I haven’t tested that.
Kobo does too. Usually it says something like "At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied." and below in the download options it will tell you DRM-free.
I don’t know a way to check in advance if a Google Play book is DRM‐shackled. Hopefully, if a purchase does turn out to have DRM, Google would let you return the book for a refund if you haven’t read it, but I haven’t tested that.