Hi
If you don't need the complexity of the Fluke or Robin type of instruments Sealand did a range of individual instruments. I had their RCDd tester and their Earth Loop Impedance tester. I have seen both of them on EBay for under £100. I also still have an apprentice training 500V hand crancked megger, I can remember the number of times I've been pushing the crock clips into the leads and someone has given the handle a turn.
Brian