The best place would probably be in the Foreign Quarter in Yalahar. There is an Elf spawn there that can be found here. You can find Elf Arcanists up on the second floor of most buildings. Also, if you do down to the dragon area there are a lot around the first loop. If you were a lower level this would probably be a good enough spawn regularly. If you are a high level you would want to wait until the rapid respawn event.
Another option would be the tower in Shadowthorn located here. This tower also has quite a few Elf Arcanists but would be best during rapid respawn.