OK, this is what has level 8 scrolls (includes Vampiric Embrace scrolls for Necros):
1. Static containers (barrels, crates, chests) when they are forced to reset, in any Tram/Fel Dungeon. If a container is a permanent fixture that can be opened, open it and remove any contents. Usually, within 30-60 minutes, it will refill with material that includes level 1-8 magery (but not the necro) scrolls, normal items, bones, body parts, and maybe gems. Strangely, blank scrolls are also a possibility.
2. Many big event creatures
3. The Dark Father in the Doom Gauntlet
4. Meer Eternals in Ilshenar, at Sacrifice
There are typically 3 families of mage/necro scroll drops.
A. Blank Scrolls up to level 3 scrolls, including the 10 weakest Necro spells.
B. Level 4-7 scrolls (including 5 necro scrolls)
C. Level 8 scrolls.
Very few creatures drop C., but typically also drop B.
Most other creatures that drop scrolls will drop Only A (ex: Imps, Orc mages, rat mages), Only B (Gargoyles, liches, and most other spellcasters), or 1 A & 1 B (Air elementals, maybe a few others).