We don't use a lot of low-voltage automotive wiring products but we found DelCity when looking for 16- and 18gauge insulated wire for a project.
They have an extensive assortment in all standard gauges used for automotive repair and offer as short as 100'foot spools in most sizes - shorter lengths in larger wire too.
Also a ton of crimp connectors, insulated tubing, circuit breakers and switches useful for automotive and marine applications.
Fast and Free shipping with $100 order - and that's a great deal because a lot of wire is pretty heavy and freight cost is a killer. Pricing is better than I've found most anywhere - I don'tknow how they manage to supply LV wire so inexpensively but I'm not going to upset the apple cart and ask!