I suspect your left channel working its way loose its a socket issue, rather than a plug issue. Sockets are far more prone to variances in manufacturing tolerances.
Third-party cable wise, I don’t know of anyone offering SR1a cables as a standard product. And of those that are willing to do it on a custom basis, a couple of their potential customers pinged me for a “how to do it” (cheeky sods …) … which doesn’t bode well for the end result …
I build and sell cables from time to time … and have three designs for the SR1a … the varying impedances of which affect the tonal profile in audible and measurable ways:
But since I’m not really a commercial cable builder, there is no scale-of-economies at work with such things, and I tend to stick to the best available components (including hand-wound complex wire geometries using very expensive raw wire), AND I don’t like to work for minimum wage, my cables are usually prohibitively expensive.