One of the oldest independent ebook stores, eBooks.com offers a massive selection of mainstream fiction, non-fiction, and academic titles. Smashwords

While Amazon is the default, you can buy Kindle-compatible ebooks from several alternative retailers and even borrow them for free through library services. Because Kindle now supports files through the "Send to Kindle" service, your options for where to shop have expanded significantly. Top Retailers for Kindle-Compatible Ebooks Why It’s Great for Kindle eBooks.com

You don't always need to buy. The Libby app is the most popular way to borrow ebooks from your local public library. Once you sign in with a library card, you can browse their digital catalog and select "Read with Kindle" to send the book directly to your device for the duration of the loan. How to Get These Books Onto Your Kindle

Rather than just a store, BookBub acts as a discovery tool that alerts you to deep discounts and free Kindle books across multiple platforms. Free Library Options

Since you aren't clicking "Buy Now" on Amazon, you'll need to use one of these two methods to transfer your files:

This is the go-to spot for indie authors and self-published works. It’s perfect for finding unique stories you won't find on bestseller lists. Baen Books