A redemption mallet is a common (but expensive), replacement part used in the arcade game and amusement industry. The name comes from their most common use on redemption machines (the games that give you tickets when you win).
Your basic redemption mallet is a big padded mace of sorts, it is more of a mace than it is a mallet or hammer. These are used in various "Whacker" games such as Whac-a-Mole, Wacky Gator, Cracky Crab, Gopher Bash, Robo Bop and a whole subfamily of Midway style games where you bash a mechanism to launch a "frog" or other critter into a target area.
These mallets come in two basic sizes. The larger size (which is most common), has a head 7" in diameter and 6.5" tall. While the smaller size (commonly used on Whac-a-Mole Junior and Wacky Gator), has a head 5.5" in diameter that is 5" tall. These mallets can be ordered from most online arcade and amusement supply vendors. Prices range from $45 to $65 depending on size and quantity (most vendors only carry black and blue, although red mallets used to be quite common). Your local coin op repair shop should also have these in stock.
Now you may want to know why in the world you would ever want to buy your own redemption mallet. The answer is simple, you can hit people with them (without causing any real damage, like padded swords, and other fun toy weapons). My old roommate used to have a pair of big black redemption mallets, and it was quite therapeutic to solve arguments with them. They are very durable as long as you don't go around bashing sharp objects. They tend to last only a few months in the arcade, but you can safely bash people with one at home for years before they will fall apart.
One small note. Don't go around bashing strangers with one of these things. That is a good way to end up in jail, or worse.