You might start here:
https://www.mobileread.com/forums/sho...11&postcount=3.
Regarding formats, if a site doesn't specifically list Kindle as a format, look for Mobi instead. In general, Mobi books can be read on a Kindle.
Personally, I obtain most of my books from the E-Books section right here on Mobile Read, and also from Feedbooks. And, of course, from Amazon.
To find a free ebook at Amazon, go to the Kindle store and search or browse in the usual way. When you've narrowed your choice, set the "Sort by" control to "Price : Low to High". In most cases, you'll see a bunch of free books at the top of the list.
Hope this helps.