For anything below 1.9
SpawnEgg egg = new SpawnEgg(EntityType.CREEPER);
ItemStack creeperEgg = egg.toItemStack(5);
For 1.9 and above
In versions 1.9 and higher, Spigot does not have an implementation for creating spawn eggs without using NMS. To accomplish this, you can use a small custom class/wrapper to make it happen:
public ItemStack toItemStack(int amount, EntityType type) {
ItemStack item = new ItemStack(Material.MONSTER_EGG, amount);
net.minecraft.server.v1_9_R1.ItemStack stack = CraftItemStack.asNMSCopy(item);
NBTTagCompound tagCompound = stack.getTag();
if(tagCompound == null){
tagCompound = new NBTTagCompound();
}
NBTTagCompound id = new NBTTagCompound();
id.setString("id", type.getName());
tagCompound.set("EntityTag", id);
stack.setTag(tagCompound);
return CraftItemStack.asBukkitCopy(stack);
}