Virtually any book store other than iBooks, Kindle and Barnes and Noble should be fine for your needs. You will need to simply make sure books are in either ePub format (ideally) or PDF format (if nothing else is available, though ePub works better). Virtually all the other bookstores offer that. A good place to look is
www.inkmesh.com or
http://ebooks.addall.com/. Both of these are price comparison web sites and they will let you shop around. Be aware that there are geographic restrictions on some books. That means that you may not be able to buy them where you are. Some stores will tell you in advance whether a book is restricted. Others will only tell you when you go to the checkout page. In some cases, you can get around the geographic restrictions by giving a fake address in the correct country, but other places have additional restrictions which will prevent you from doing that.
I also recommend Calibre. It is a free program which will help you manage most of your ebooks. It will *not* work for books which have DRM (digital rights management) on them. For those, you need either the Sony software or Adobe Digital Editions, both of which are free, although both of them are poor choices compared with Calibre. Typically, when you download a book from a book store, you'll know it needs either the Sony Software or the Adobe software because the extension will be .ascm. If the extension is .epub, you should be able to use Calibre.
I hope that makes it clearer. It isn't too hard once you get used to it, but I know it can be a bit overwhelming at first.
Eric