You can extract rootfses from update files using
NiLuJe's KindleTool. as for updates you can grab them straight from Amazon servers, even older versions. For example at the time of writing this is the latest FW for PW3:
Code:
https://s3.amazonaws.com/firmwaredownloads/update_kindle_all_new_paperwhite_5.15.1.bin
You simply need to change the URL link to the version you desire, like
Code:
https://s3.amazonaws.com/firmwaredownloads/update_kindle_all_new_paperwhite_5.12.2.bin
This will work down to roughly 5.8.x range as that's when Amazon started using the new links.