IF you decide to get your steel from the blade of an old lawn mower, be sure to FIRST REMOVE THE SPARKPLUG. Manually rotating the blade by hand can cause it to "whiz" around faster than you can get your hand out of the way. It's a good secondary safety measure to wedge a piece of wood (like a 2X4) to keep the blade from moving at all while you work on the crusty nut.
If you look up lawn mower repair shops or vehicle suspension install shops in your area, they may have old lawn mower blades / leaf springs lying around for a very reasonable cost...maybe free.
Good luck, and be sure to post pictures of your progress. Although you may not get free steel here, you can get lots of free advice.