The Fluke 80k-40 40kV probe is for measuring voltages up to 40kV. It is for use with an AC/DC voltmeter or DMM. It has high accuracy when the voltmeter or DMM has an input impedenance of 10M ohm +/- 1% but can work with other impedances (you just have to do more work). Luckily my voltmeter meets this very requirement. You can see it in use here in testing my 30kV high voltage power supply and again here in testing my negative and positive high voltage power supply.
It's made by Fluke but I purchased it from a local electronics store. It shouldn't be hard to find as Fluke is one of the biggest name brands you find in these stores.
Video of the Fluke high voltage probe in use
Here's a video I put together showing me using the probe to measure the voltage used to fly a lifter. I also take the probe apart to show the insides.