The other way to purchase eBooks is to shop online, either at the website of the store that supports your device or at a website that sells eBooks in a variety of formats, like eBooks.com.
Before you can purchase eBooks online, you will have to have an account with whichever service you choose.
When the account is set up, you can then start purchasing eBooks online.
Many who get the devices don't realize that bookstores typically make their eBooks specifically for their devices, and as such, each can offer different prices, availability and more.
Currently, eBooks sold on a bookstore website will only work on the device(s) supported by that store.