On spring hangers, nothing special.
Just a piece of ordinary mild sheet steel (1/8" or 3/16"?). Available at any shop that sells misc. metals. Sometimes these shops have a shelf or bin of scrap pieces that can be had for cheap. Could cut with a hacksaw or cutoff wheel in an angle grinder (or a few other methods) to match the existing dimension. The steel shop should also be able to cut a square piece for you on a chop saw or band saw. Use a file to smooth off the cuts and drill holes to suit. Course, some welding required to frame.
Spring hangers are sold separately like at easternmarine.com here for example.
If you want simplest/fastest, go to a frame and axle shop.