The Internet is a wild and wooly place, a vast untamed wilderness of all things wonderful (and not so). If you are looking for good quality, not ancient, and formatted with care for your Kindle (and other ereaders), you can get lost really fast. There is a lot to choose from and not all sites are trustworthy.
Use Calibre (you have installed it and are making friends with it by now, right?) as more than just an ebook organizer and quick reader. It has a button in the menu bar named Get books that, by clicking the little down arrow to the right of it, will give you access to quite a few places to browse, many of them free. Notice manybooks.net is one of the listed places. This is a great place for you to click and browse and get to know some of the things Calibre can make easier for you. The Stores area is where they are all listed. If you move your mouse pointer down a bit and click the words Choose Stores, you can deselect the ones you don't really need or want. I want to at least explore all the various links to these stores and will probably end up removing the ones that are in languages I can't read, or countries I can't shop in. I noticed the Amazon (US) link caused my default browser to open with the Amazon Kindle store nice and handy for me. I'm thinking if you purchase a book, have it sent to your Kindle normally, then the next time you have your Kindle connected to your computer, turn on Calibre and copy it to your Calibre library too. This list is very interesting, with most of these "store" totally new to me. I'm going to enjoy exploring!
The way this software is so frequently updated, I wouldn't be surprised to see more "stores" added as time goes by. This is certainly an area to keep an eye on if you are interested in new places for browsing reading material.
More on Calibre tomorrow...