On XDA, in the user profile of Astarioth, there's a link mentioned in the "most thanked" tab: https://dl.dropbox.com/s/vprplelu9zx7s2o/kindle_fire_usb_driver.rar?dl=1 (and this is the XDA post thanked for). The name of the .rar
archive suggests it holds your wanted drivers -- and the quote
[...] uninstall your old Kindle USB Driver and install this one.
confirms my guess :)
Now, why didn't you find it in your SDK directory, you might ask? Guess that's as it goes with apples and peaches. You assumed Google's Android SDK -- while Amazon has its own Android SDK. And that line most likely referred to the latter, while you have the former installed. Also only a guess -- but I'd say I guessed right again? You might want to read Setting Up Your Development Environment, take a special look at the section Installing SDK Add-Ons. If you work with Eclipse, this gives you another source to get to the Kindle drivers.