You can get them from killing mobs. It's very rare to get so good luck. Also you can only get the head of the mob you kill the mob you want the head of (EX kill a zombie you can ONLY get zombie head). Also it is impossible to get a player head unless you spawn it in creative or get it in a bukkit server.
You cant find a giant mob in vannilla minecraft but you can sumon him using a command or a spawner.
In Minecraft, a Wither is a hostile mobile game entity which can be spawned by the player only in Nether Fortresses. It is skeletal in appearance, has three heads and floats. Fighting the Wither can be difficult, and it is considered a boss enemy.
Go to minecraft wiki and do a search for "mob grinder" and it gives you a tutorial of how to do it.
Nobody knows. The heads of the Mob keep a strict confidentiality, because if anyone were to find out, they would land in jail for life.
no
a bat
Mobot
by using spawn eggs or mob spawners with a mob in them
a monster mob in minecraft
witherboss
The first mob in Minecraft was the Human, which could be spawned by pressing G and looked like Steve. It was later removed in Classic.
You can get a mob spawner in Minecraft by using any pickaxe (except wood) with Silk Touch 1 enchantment or by spawning it in (Dec:52).