Only played solo so far and not playing tonight.
Found what I needed. Spoiler :
The Flayer Dungeon, where the brain drops, is accessed in Upper Kurast.
This is a good link, but lots of spoilers there, if you don't want to explore for yourself which areas have changed.
Eastern Sun map layouts
Note there is a branch of brand new areas from the Ancient Tunnels, which are for high level, tough characters. Niiiiiice.
There are 16 added areas, where the best drop rates and xp are, plus the Cow Level. You are advised to go to these after completing the difficulty setting. You'll know when you enter one as mobs are bloody tough; they are spread out through the 5 acts, so you might wander into one or two. They are listed at the top of the map layouts, linked above.
This is also worth a read :
ES Playing Tips