If you can forge it out (power hammer, forging press, or a hell of a lot of hammering) a leaf spring would be good stock for it. Unless you're looking at new steel - in that case, I'm not read up on it enough to give a GOOD opinion.
If you go the leaf spring route, I recommend finding a local scrap yard and pulling them off a car or truck. If you need bigger stock, find a big truck, like a dump truck or a semi. Those springs are usually a LOT bigger. Severe duty trucks like trash trucks usually have MASSIVE spring packs, as well.