If you're caballed you can often ask higher level cabalmates to leave a corpse for you when they are ranking, exploring, getting preps, etc. You can also animate corpses from PKs.

Generally speaking, the higher the mobs HP the better, since zombies tend to soak up damage. Factor in convenience and ease of killing and you've got your formula for which mobs. Like Daurwyn said, a lot of necros go for war-wearys and other summonable mobs because they can fight from a safe place (like a graveyard!).

I've never played a necro above 30 because I think building an army would be tedious and boring even on a good day, and losing them to a summoning paladin or something like after an hour of work would put me into a german-gamer-kid style rage.

If you tend to play long sessions you might enjoy the class quite a bit. After observing a couple necros recently, I've come to learn they're better at exploring and mob-killing than I originally though, even the non-undead ones.