I use a 6' long 3/8" T&T tools probe. Ground here in Texas is hard a bricks so you need a good solid probe...their probes have replaceable tips so I've been running the same probe for the past 6 years but probably gone through 10 tips in that time.
One option to consider is from an automobile junkyard. The rod which was part of the trunk spring in older cars is spring steel. I recently got one, heated and straightened one end, used a 5/16 " die to thread it. Placed 2 nuts back to back, tightened against each other to prevent loosening. Ground nuts to a taper. Placed a piece of fuel line over other end for handle. Been using probes for over thirty years. This is the best yet.
This is where we got ours they are great hand made spring steel tip and handle are amazing. I would definitely recommend they used these on the history channel on a digger show. They definitely sing when you hit glass or a bottle. The craftsmanship is very important and a hollow handle so it sings and vibrates you also want a well built one so it doesn't break and you impale yourself. They are around 68.00 shipped for a 5 foot and that's what I would recommend.