Flexible wire for test leads

If you need really high voltage insulation on the leads, the old timey non resistor stranded spark plug wire works pretty good, but a bit bulky.
 Wildcat- I have seen it offered on Ali Express very reasonably.

 Mark Oppat at Moppatatcomcastdotcom likely has some available. Just add the appropriate characters to the address and delete the alpha subs.

Belden is also a good source for test lead wire to make your own test leads. High grade standard test lead wire is still made with rubber insulation, is rated for 5KV working voltage and gives good service for a long time. 

I did a quick ebay check and at least one seller is offering Belden 20 Gauge wire with rubber insulation in red and black 10' lengths for a total of 20' for $9.90 plus $3.95 shipping; and another seller has 20 Gauge wire, red only, 25' with special plastic PVC insulation at $17.44 post paid.

As for finding some shielded wire with a braid over one or more conductors: Coaxial cable such as RG-58 has been used as a substitute. Coaxial cable with a solid center conductor will not be very flexible. There was quite a lot of shielded wire (cable) available at one time for audio, video, microphone, and use in receivers and transmitters where it was needed. Some of it had special properties. There should be some rolls of NOS shielded and stranded wire out there that is more like what has been used in radios and test equipment. If worse comes to worse, you could salvage small amounts out of junked equipment. 
73, Rod  WB6FBF

Pomona is a good quality online source. Look for Silicon rubber with a high wire strand count (the more stands make it more flexible) I think I have see some with over 40 strands, several gauges of the wire itself. Lots of sellers on Ebay too.

Where can I buy flexible wire for use as test leads?  The big shielded wire used in signal generators, scopes and that?  Maybe I don't know what it's officially called besides "flexible wire".  I can't seem to find any for some reason.  Thanks.