Kobo.com has a mixture of DRM and non-DRM ebooks, but their own search doesn't let you choose just non-DRM.
However, you can work around this by using any search engine with the terms:
(search terms for book, author etc) "drm-free" site:kobo.com
I wanted to use DDG to create a simple search box to do this automatically ( https://duckduckgo.com/search_box ) but I can't work out how to add the "drm-free" bit.
Can anyone help? Doesn't have to be DDG search.