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.