If you find a dead link when you're trying to find a source, you can try a few other things.
If the link, video, or image looks like it has a filename that came from RedGIFs (ThreeRandomWords) you can try the following link formats in an archive service.
https://gfycat.com/[ThreeRandomWords]
http://www.gifdeliverynetwork.com/[ThreeRandomWords]
https://www.redgifs.com/watch/[ThreeRandomWords]
Gfycat banned NSFW content in 2020 and moved everything to RedGIFs if the user was active and chose to move it over. If the account was not active, or the user did not choose to move it to RedGIFs, it was moved to GIFDeliveryNetwork.
If you find a PornHub username name you can try doing the same but with these link formats.
https://www.pornhub.com/channels/[Username]
https://www.pornhub.com/users/[Username]
https://www.pornhub.com/model/[Username]
https://www.pornhub.com/pornstar/[Username]
If the link looks like it might have been active not too long ago, you could also try finding a web-cached version.
Archives:
Wayback Machine -
https://archive.org/ - Enter the URL in the search bar.
Archive.is -
https://archive.is/ - Enter the URL in the search bar.
Megalodon -
https://megalodon.jp/ - It's all in Japanese, but just enter the URL in the search bar.
Web Caches:
Google - Search the URL in Google, if the account comes up click the three dots to the right of the URL, in the popup, click the down-pointing arrow, and click "Cached".
Bing - Search the URL in Bing, if the account comes up click the pointing triangle to the right of the URL, "click Cache".
Yandex - Search the URL in Yandex, if the account comes up click the three dots in the top right of the search result, and click "Saved copy".
Yahoo - Search the URL in Yahoo, if the account comes up click the pointing triangle to the right of the URL, "Cached".