Mostly Kobo, as they are willing to sell to me even if i'm in france
Amazon does too, but i can't be bothered to convert to get a lesser quality ePub in the end. (Sorry amazon, you showed up a few years too late...)
Sony for the books that still happen to be geo-resticted on kobo, as geo-restictions can be bypassed.
Fnac and ePagine fro french books. Smashword and feedsbooks too.