There are items all over Suramar that can increase the maximum amount of Ancient Mana you can carry by 100. So far I have found two of these items.
Volatile Leyline Crystal — In Falanaar, enter the Temple of Fal’adora. You will find a crystal at the Shattered Locus. You need to go downstairs where 4 Shardmaiden are casting around a circle. The crystal is in the center of the circle (unless it has random spawn points). There is no exit in that room other than the stair you go down through.
Kel’danath’s Manaflask — The flask can be found in Koralune Manor near Kel’danath’s building. The flask is on the bench.
This guide will be updated as new items are found. Based on the achievement “Why can’t I hold all this Mana” there are 10 items around Suramar to increase your Ancient Mana Max up to 2000.