The easiest and most lifelike household item is the rice bag breast pad.
Take a nylon then fill it with rice. Usually a cup will do for a B cup presentation. Tie a knot in the nylon then place in your bra. Not only will you get a nice shape, but the mass and movement will be very lifelike.
I think if you google it you will find a table that will give you an approximation for the amount of rice for a particular cup size, but you can just use your judgment.
-Sandy