Is it still possible to pick this quest up somewhere?
Think you can still pick it up in Aegwynn’s Gallery in Legion Dalaran.
nope, not there anymore sadly 
Pretty sure it’s in Stormwind for alliance and Org for horde
Try using the Adventure Guide. 
Yep, Nash was correct. Locations are here, although you may need low level quests tracked if the exclamation mark isn’t already present on your map: https://www.wowhead.com/npc=221491/khadgars-summons