Primarily Kobo (only if there are discount codes that I haven't already used), but also Baen and Smashwords. Been looking into Google Books lately, too. The only outlet I categorically don't buy e-books from is Amazon, but that is just me being stubborn.
I used to buy e-books regularly from iBooks (Apple), but when I could no longer scrub my purchases, I stopped and shortly after switched from iOS to Android.