https://www.libreture.com/bookshops/ has a list of stores (and for Norwegian books there is https://ebok.no/ ), but I have found no way to search "globally" (across sites like that) for DRM-free ebooks.

https://news.ycombinator.com/item?id=32442549 has some more tips.

Oh and https://bookshop.org/beta-search now actually lets you check "DRM-free" when you search, which is a very good step in the right direction!

I have a dream of https://news.ycombinator.com/user?id=marginalia_nu taking up the mantle. Though I think anyone could make something that's better than what we have now, which is nothing.