They're all up on AWS S3. You can find them by going to the official firmware update page, clicking the one that corresponds to your device, and finding out where you end up redirected. From there, it's easy to change the URL to the version you want.
For the PW4, the update link redirects here:
Code:
https://s3.amazonaws.com/firmwaredownloads/update_kindle_all_new_paperwhite_v2_5.14.3.0.1.bin
So, for 5.14.2, you'd want this URL:
Code:
https://s3.amazonaws.com/firmwaredownloads/update_kindle_all_new_paperwhite_v2_5.14.2.bin
And sure enough, that's where you can find it.