I've started to rely more and more on the Kobobooks site not because of ease of use (subpar for many reasons) but because of the frequent discount coupons (check the sticky in the "Deals... "forum), which allowed me to make quite a few remarkable catches.
Besides, any store selling epub books will do for you - I don't know where you are based, but I think Google books is available in most English speaking countries. In the UK major supermarkets, too, sell books in epub format, and you can generally also buy direct from the publisher.
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