I've not done it, but the first thing that comes to mind is old leaf spring material. Here there's a 4X4 shop that has a bone yard of leaf & coil springs that have been removed from Jeeps, trucks and buggies that are getting suspension upgrades. They sometimes say "take what you want", but might also expect scrap metal pricing. Seems to me you could heat with a torch to form them to fit, then heat them again for quench to restore the temper.
Another option might be to find some brackets from old iron and heat to form, then re-temper them. Maybe MikeP has ideas?