You can get Zombie Key from every 10 wave boss in endless mode, or buying for robux. Or easier way, get keys from pass.

Units you can get from Zombie key:
52% - eye ball

39.7% - cauldron toilet

5% - Vampire Camera Woman

3% - Pirate Speaker Woman

0.3% - FrankEngineer

0.01% - Zombie Titan Tv guy