In person, a West marine store.
Marine Wire | West Marine
The larger stores have more variety of insulation colors. Marine wire has higher temperature rating, sized to AWG rather than SAE, and has finer tinned stranding than SAE.
Mail order, less color choices but will sell small quantities Also has wire for use in engine bay that should have higher heat and oil resistance than general purpose primary wire.
[URL]https://www.wirebarn.com/Wire-Selection-Guide-_ep_29.html[/URL]
Small company. I've bought a few things from them such as terminals for headlights and they took care of a mistake right away.
Seem to carry quality stuff. Also will sell small quantities.
[URL]https://ceautoelectricsupply.com/individual-components/automotive-primary-wire/[/URL]
One more that's been posted here but I can't say I've dealt with personally.
[URL]https://www.wiringdepot.com/wire-bundles-automotive-primary-wire-8-30-cuts.aspx[/URL]