I use mainly Amazon, with the occasional purchase from Baen and sometimes an authors website. I haven't bought from Kobo in some time. I would like to but I find them second rate compared to Amazon. Their prices are also too high. And yes, I know about the price match, but wny would I go to the hassle for every book I purchase. Amazon simply does it better. For Public Domain I use the usual sites mentioned here, though Standard is new to me. Thanks Zod! One PD source not mentioned here which I find useful is the following:
Adelaide University EBooks
So far as I'm aware this site includes books which are in the Public Domain in Australia. Some books available here may not be in the Public Domain in your Country.