I found the link for the Kindle fonts:
http://www.orangetype.com/kindle/
Ignore the comment that you need to jailbreak your Kindle. I think the webpage must have been written before some lovely person discovered USE_ALT_Fonts.
I don't use calibre to sideload my fonts. Everything I do is through Windows Explorer. I connect my Kindle to the computer using the USB cable then I copy and paste the ebook files into the documents folder on the Kindle. I delete ebook files this way too.