Zombie Party is one of the three skins released under Zombie Party offer. It could be purchased as part of an item deal for a limited time in early August 2017.
Zombie Party could be purchased for $19.99 in the "Best Deal!" which included the Walking Hand skin and either 50000 coins (Browser) or 1800 DNA (Mobile). It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: The Zombie Party sub-offer was released again for a limited time in mid June 2019. This time the price for the Best Deal was lowered to $9.99 and the included coins were reduced to 30000. The package of DNA was no longer an option on either platform.