Not sure where you could buy one. You can make one fairly simple if you have the tools. Go to a car wrecking yard and ask for a trunk lid tension rod off of an older car. Thread the one end for a handle and round the other end for the probe tip. Good luck finding your privy!
They are utility probes but work very well, especially if you have fairly compact top soil. I've used 1 probe for 6 years now, though I recently bought a backup as my original is getting a little thin (I use the 3/8" hex rod). They have replaceable tips.