I use a plastic tipped mallet. I'm guessing its 4 to 6oz. Its as old as I am and I don't know where it was purchased. This works well on molds up to 4 cavity. I don't have a mold larger than that.
I sometimes cast with 2 other friends. One of them uses a hickory tool handle. The other settled on a plastic tipped hammer close to the weight of mine after he tried several other things.