Check out squires, you can order anything you want from them, I've ordered buss wire, solidcore , great prices and the quality is top notch..
They're great people...
12GA romex works fine, my only complaint with using the raw romex wire is, it will tarnish like a penny quickly since its raw copper and when your working on the amp all the skin oil which gets on the wire accelerates the tarnishing, I guess you could in theory seal it after the solder work is done so it doesn't oxidize/corrode.
As for soldering to any thick wire, MAX heat on my hakko 936 and some Flux pen action works every time, bus wire, chassis, fender ground blobs..
Nice work Jelle