Don't know about Calibre? It's a free library management system for any and all of your e-book needs. In order to use it, you'll need to remove your Kindle's DRM (digital rights management), which you can do easily. Once you have Calibre installed, you can convert your books into any font of your choosing.
See how to do it here.
Or, if you don't want to go through Calibre, you can add alternate fonts after connecting your Kindle to your computer. Once you have your fonts, you can drop them into your Kindle's root directory.
Learn more here.