Necrowart Zombies can be found mainly in Briarwood. Their appearance isn't much to be desired as they walk on all fours, frequently wheeze, are somewhat purple in pigmentation and have a mushroom growing out of their back.
Necrowart Zombies are mainly found wandering near the stream in Bristlewood, Briarwood and require a level 9 combat job to encounter. They also feature in some dungeons such as the Haunted Mines in Blackspore (which requires a level 13 combat job to enter) except they are reffered to as Shamblers and do not have a mushroom growing out of their back. They also feature at Halloween when the Pumpkin Prince arrives. Sometimes the Pumpkin Prince will spawn a spawner wich will spawn hostiles such as Grave Elementals and Necrowart Zombies.
Also if you own a level 11 or above farm you can purchase a fungus sprayer from the vending machine on your farm. This enables you to turn yourself and others into Necrowart Zombies. Also at Halloween if you are attacked by the Pumpkin Prince ther's a chance that you will turn into a Necrowart Zombie and lose your ectoplasmic shooter and will have to pick up another one once the effect has worn out.
A Mini Necrowart Zombie variation can be bought in the Coin Store as a Pet Pal during the Super Spooktacular.