I’m looking for a store where I could buy e-books which are usable by Kindle. I don’t use the e-reader that much, so I don’t want to buy a completely new device as the old one is perfectly fine. But I also want to have an option to use it from time to time (Amazon already got their money so it would be a waste to buy a new device)

My main problem (and please correct me if I’m wrong) is, that stores which I found mostly contain books in epub format (with DRM) which can’t be opened by the Kindle and which would need to be converted via 3rd party applications.

Are there some good stores with ease of use that’s similar to the Amazon’s one? Buy a book, send it to device, done. Or do I need to accept, that I will need to somehow convert books before using them?