Official Stats are in Volo's Guide to Monsters
Volo's Guide to Monsters (released November 2016) includes many additional Illithid variants and related creatures. One of these is the Elder Brain.
The official Elder Brain description can be found on pages 173 and 174 of Volo's Guide and includes lair actions, regional effects, a full stat block, and lore on the nature of the Elder Brain.
But Matt Mercer's Elder Brain is homebrewed
Matt Mercer's Elder Brain appeared well before the publication of Volo's Guide, and thus is distinctly homebrewed. Additionally, Matt himself has stated:
...and at full strength [the Elder Brain] is a CR 20 creature.
This doesn't line up at all with the official publication, which only has a CR of 14. A quick search didn't turn up any exact stats for his Elder Brain, only a meager collection from the third party critical role stats site which has the following information:
- Elder Brain First Appearance: 10 K’Varn Revealed
- Armor Class 18 (at weakest), 20 (before party flees)
- 320 damage taken and “not on death’s door yet”