Here is a list of known Necromancer Trainer locations:
Aghram
Manvera Khani - In a Building (-29930, -19482, 1824)
Arena
? - The Arena in Qalia - The Arena (?, ?, ?)
Arlinora's Retreat
? - Thestra, SW of Veskal''s Exchange, right by the Lyceum (?, ?, ?)
Dahknarg
Baelar Shane - Ghostclaw Lair (15191, -30593, 17731)
Dragonweir
Kolandru Farihn - Dragonweir Fortress (-75837, -76949, -11938)
Hathor Zhi
Qaeria An'jiir - The Dark Academy (-75141, 16713, -10576)
Kasawari Gulch
Behbaba Dibn - Onboard Ship (-24114, 25287, 1185)
Khal
Dulmier Shurdelra - Arcane Guild (-19502, -65945, 1600)
Lomshir
Karaja Asasagras - Arcane Trainers (7252, -11115, 7461)
Martok
Hexxik Zarpel - Zarpel's House of Undeath (24638, 50066, 7025)
Mekalia
Ulwa Tebix - Uebatix's Institute of Wizardry (54454, 19037, 2925)
New Traganor
? - Near the King (?)
Tanvu
Veora Lan - Trainer House (45937, -21396, 4569)
Tursh
Garret Belant - Western Watch Barracks (71745, -15033, 37762)
Any others out there? Probably, but if you know one please post the new info and we'll add it to the existing list asap. Thanks!