Where is (possible spoilers):

[milo]

The elf is in a building north east of main castle in thousand rivers.

The barkeeper and brewery that you complete the quest for gathering plants are both in buildings on the north of the arden peaks map.

Look at your map for upper hadar, there is a area kind of north east that cannot be reached from upper hadar, you should be able to see the submarine and water from the other side of the wall. Just go down to lower hadar and up the corresponding elevator, it is in the north east.