There are plenty of ebook stores if you google around, that have a standard range and use Adobe DRM, so off the bat wouldn't work on a kindle. In theory you can remove that DRM using Calibre, but I haven't tried.
Other than that, not really? There are plenty of ways in which you can buy _a_ drm free book, but not some large range (or even bandcamp-quality range, where there are authors you've heard of, just not Dan Brown / Stephen King sized ones) site.
I haven't got around to solving this problem so am also interested. I already own a kindle, I don't want to generate ewaste by changing physical device.
Was impacted by invalid ASIN pop up. Got fed up finally. Sold my kindle paperwhite via classifieds to someone who would embrace the Amazon walled garden (so someone new e-reading). Then bought a pocketbook. Now all my ebooks work again. And no waste. I use beam ebooks for drm free books. Bought all my Expanse books there.
I own an Onyx Poke and I buy my books at kobo. They work just fine. With a jailbroken Kindle capable of processing EPUB this should (!) work. Give it a try with an inexpensive book, maybe?
I buy ebooks outside Amazon with my non-jailbroken Kindle. They’re usually DRM’d EPUB files, which I unDRM with Calibre and transfer to my device, which I have kept in airplane mode for the better part of a year.
The bonus part is Calibre keeps a local copy on your PC, so now all my book purchases are backupped and without DRM.
Borrow from the library! Kobo has library borrowing built right into the device if you're in the US and can use Overdrive/Libby. Otherwise, you can use the Libby app to send the book to your Kindle.
I want properly created files the kindle can render with the options I want, not a pdf that forces a layout.