You can only get those antlers by casting an antler spell on the Big Zombie Deer at Halloween. So you will have to wait until then to do it.

Since no one mentioned it,

The BZD (Big Zombie Deer) only appears in October, not just Halloween. He shows up throughout the month.
To get the Skull Mask you must cast the Skull Spell on him
To get the Antlers you must cast the Antler Spell on him
And to get the Pelt you must cast the Pelt Spell on him.

Due to the fact that everyone likes to bunch up ON him when he arrives you might need a bit of patience to get the things you want. So the only major way to get the Zombie Antlers is in October when BZD shows up, or if someone has the Spell Data to send to you.