Hello. Here are the areas I can think of off the top of my head.

Galadon graveyard
Crypts under Arkham
Mauseluem (NW corner Galadon GY)
Keep of Barovia
Eil Sherea
Skeletons on Mountain North of Darsylon (Caladryl?)

I think the spirits under the sea of Despair might be undead also, but I am not sure. I know a necromancers disruption hits something down there. Also note I did not put levels on there. If you don't know these areas you do not want the Maus under 27 (varies by class but about that), the Keep under 40, Eil about 40 for ghasts and higher for undead guildmasters, and damned near hero for Skeletons.