@stephendunn I did the 14ga tempo electric wire. No marketing, no fancy terminations, just .9999 pure silver in an oversized teflon jacket (so that the wire is mostly just floating in air for the best dialectic rating possible). They charge around $200 per meter for stereo pairs and give you a 30-day trial period.
Basically, no-one's going to give you better raw materials, they're just going to mess with the dialectic (probably for the worse) by changing whatever the wire is embedded in and then change the cable geometry (which will necessitate going from solid core to stranded cables). While I'm sure things can get better, I suspect it's going to cost you $1000s to find out.
I've also experienced success with VH Audio's .99999 solid core silver in cotton for a DIY interconnect, and Ocellia's silver reference interconnects, which I bought on the recommendation of @charles1dad . In all cases, the silver was better than the copper I had in before.