My approach has been to purchase books from bookshop.org (or directly from the author if that is an option), and then immediately go find a DRM-free backup copy to send to my e-ink device.
At least they appear to be partnering with Kobo "later this year" [1]. I've been a big fan of Kobo's devices so this is a nice plus. (I just wish they could figure out some way to get Kindle exclusives, but well that's a contradiction in terms, so...)