You'd be surprised. Tor and Solaris both offer DRM free books on Amazon. Also anything self published tends to be DRM free.
I saw the writing on the wall and downloaded my books from Amazon a few months before their announcement. Out of around 1000 books I had 300ish that were DRM free.
Dumb question, but: is there a way to find/filter ones that are? (I can't seem to find anything in the (web) UI that makes it clear which books are downloadable.)
There wasn't when I went through my collection. Reading the announcement from Amazon it looks like the existing DRM free books will not be automatically flagged to be downloadable.
The publisher/author will have to go through a process to have their books be downloadable again.
I have some tor books, but I used to download them as .azw even though they had the "this book is drm free ..." blurb at the beginning. (was back before amazon stopped downloads)
Now they could actually be distributed as unencrypted .epub