McMaster-Carr supply has 3/16" x 36" rods for $1.88 each. These are hardened and would prolly hold their shape well.

Or, soft steel .192" diameter wire, shipped in a coil, is $151 for 1000'. But that stock would have to be straightened, cut to length, and bent into a hoop. And would probably be prone to rebending during insertion into the ground.

Anyone have a good corporate discount with these guys?