I dealt with the metal probes today. I don’t what kind of steel they are, but they are most definitely steel. I broke one trying to bend it in two places so that I could fit it into a project, and thus had to find something to replace it. After a few hours of testing, I have learned that A) length doesn’t matter it seems as far as different lengths, longer, and up to three inches shorter, and I replaced the probes with galvanized steel rods. They were “ties” for chain-link fencing, available at Home Depot and Lowes. They are about one gauge larger than the rods included with the kit, so you’ll have to file them down to fit.
I hope this helps somewhat.