So far I’ve found most of what I’ve been looking for on www.exlibris.ch, though www.orellfuessli.ch seems quite nice as well. Normally it says in the details whether or not a particular book comes with DRM.
The DRM-free books are still digitally marked - Ex Libris will include your e-mail address in one of the first pages, and there are probably subtle differences in the text itself. I don’t mind that, though I blame the technology for the occasional annoying ‘typo’ in the book.