Sometimes ago I realized I could only measure resistance to about 1-2 GOhm with my best multimeter. This was completely unacceptable and I decided to correct the situation. I looked quickly at the cheap insulation tester from China but decided against it, because of the limited range and accuracy. I ended up finding something a lot better but also heavier
Got myself a General Radio 1644a. At the core, this is a self-contained wheatstone bridge designed to measure high value resistors. I can now measure up to a Peta Ohm (1000 Tera Ohm)
The device can also be used for insulation testing and test capacitor for leakage.
The interesting part about the Genrad 1644a is the self-calibration feature. You don't need super expensive equipment to calibrate and adjust the instrument. I serviced the instrument according to the manual and everything is now in specs. The main rheostat is quite impressive and can be adjusted with its 8 adjustment screws.
Measuring a Ohmite 200 GOhm 5%. Was reading 205 GOhm on the 1644