The original Herobrine creepypasta (probably) came from 4chan but no one's ever found any archive of any actual threads from the time period where he's mentioned, or any earlier threads with earlier similar creepypastas that were reposted on the Minecraft Forums.
Based on old tweets and google search data it's believed the Herobrine story was originally posted online slightly before August 19 2010. In the preceding weeks similar creepypastas about white eyed minecraft characters were also appearing on 4chan, the earliest known being the "white eyes" story reposted on the Minecraft Forums on July 31.
Both the /x/ paranormal and /v/ video games boards were known to have had these Minecraft creepypastas being posted on them during this time period, so archives of /x/ and /v/ threads from July-August 2010 would be where these threads would most likely be found.
Some links:
https://www.minecraftforum.net/forums/archive/alpha/alpha-survival-single-player/814622-13-ogg-a-work-of-fiction-updated-nov-7-twice?page=3#c54
https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/114746-i-cannot-take-it-anymore
https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/115039-horror-in-minecraft
https://web.archive.org/web/20221118014621/https://twitter.com/TacticsNNiblics/status/21587389080
https://web.archive.org/web/20100903055441/http://www.minecraftforum.net/viewtopic.php?f=35&t=29828&start=30