You need to explain your particular application/test setup to get a better answer, but here's my take on the whole triax situation:
First, not all "triax" is made with the same measurements in mind. You basically have three classes of "triax" cables -- some of which are actually twinax terminated with triaxial connectors: mil-spec/aerospace RF applications, mil-spec/aerospace networking, and low noise / low leakage DC measurements (picoammeter, electrometer, nanovoltmeter, etc.).
Due to the military industrial complex's use cases, you have both 50ohm and 75ohm triaxial connectors primarily made by Trompeter/Cinch/Bel and Specialty Connector Company. You then also have custom parts from Keithley and HP/Agilent/Keysight that don't necessarily conform to the mil-spec parts and may not mate properly with them. Older Keithley equipment used Specialty Connector Co 2-lug connectors and later on 3-lug from both SCC and Trompeter. Their newest connectors, however, do not match anything made by Cinch/Bel and are likely custom. HP/Agilent/Keysight connectors have always been custom made by their YHP branch in Japan and seem to mate okay with the various cables I have on hand.
The Triax for RF applications is absolutely unsuitable for measuring small currents with a picoammeter or electrometer. For example, just picking up a Pomona 5223 cable (using Beldin 9222 triax) will cause an overload condition on the 2nA range or lower. Beldin 9222 cabling should be avoided when and where possible. The twinax cable used primarily for mil-spec networking can actually be okay to use if you don't require a proper guard connection -- as is the case for most low current measurements. These cables often use fluoropolymer insulation and are reasonably priced for the relatively good performance they offer. Many photo diodes use this type of cable, like Newport's 818 and 918 series silicon photodiodes. So don't ignore twinax as a possible candidate for low current measurements.
Keithley and HP/Agilent/Keysight triaxial cables have very low triboelectric noise when flexed or hit. They also have very low DC leakage and are reasonably flexible. Buying used on eBay is likely the only place where you will find these cables for a reasonable price.
The Keithely SC22 cable has a black PVC jacket with tinned copper outer shield, bare copper inner shield and bare copper center conductor. The inner dielectrics are polyethylene, with the inner-most having a graphite coating between it and the inner shield to lower charge generated between the center conductor and inner shield when flexed. Characteristic impedance is 40 ohms between center conductor and inner shield and 15.3 ohms between inner and outer shields. This cable is suitable for use with both nanovoltmeters and electrometers.
Standard coaxial cable like RG-58 can often times be as good as most triax (even Keithley and HP triax!), but you will have to test the cabling to make sure. To test, connect your cabling up to an oscilloscope with the input set to 1Mohm and on the lowest vertical range (1 or 2 mV/div). Next, set the trigger to single-shot or equivalent and the threshold to a little above zero (where it won't false trigger) and then hit or flex the cable and note the peak-to-peak voltage. Induced current is then 1 nApk per 1 mVpk into 1 Megohm. You will likely have to use a current amplifier such as a Keithley 428/427 to measure lower noise cables from Keithley and HP. Note: do not use Pomona 4964 SS "low triboelectric" cable. It often times performed worse than "standard" RG-58 cabling in my lab -- which is pretty ironic.
If you are making a custom setup, and do not require the use of triaxial connectors, you have a lot more room for making a low cost, high performance test setup. Coax is just fine to use inside of a fixture, were the shield can safely be at guard potential. You can also use twisted pair cables made with fluropolymer insulation. Triax is not the be-all, end-all of low leakage measurements.
Finally, as pointed out by shodan@micron, there are also Russian made triax cables that have even better specs (at least on paper) than those made by US/Japanese manufacturers (even Keithley and HP/Agilent/Kesight). Sadly, Shodan has deleted most of his posts on the eevblog forum. But I think he has a personal website with the relevent details if you are interested in perchasing some. It might be much cheaper for you to buy Russian made triax than US cabling if you are in India.
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