We use one on our Hustler 37 in. Its a factory weight that fits on top of the deck support between the front axle spindles. It weighs 35 #s. You should be able to custom make one for your make of mower if a factory one is not avaible.
topher924, eXmark has some for WB's. Weigh 9.5 lbs. each. (if you ever got some Zero Flat caster wheels/tires, they weigh quite a bit). Think Homer has a good way to weight his down.
Link to eXmark (last item on the page) http://www.exmark.com/accessories.htm