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Studio miniatures does a plastic zombie sprue, it's quite limited 4 bodies with 8 head options and 6 pairs of arms, but pretty cheap though
I know you said plastic, but don't discount metal and the amount of time you having remaining.Victory Force sells metal zombies, with many many varieties. $2.50 for a zombie that you don't have to spend 20 minutes gluing together is nice. The plastic zombie may only cost $1.25, but calculate how much your work pays you an hour. A $1.25 zombie could cost $11.25 if you count the time investment. Our time on earth is finite, before we become sleeping dead.