I buy mine at garage sales over the years. I have large spools stranded of most any gauge, 12, 14, 16, 18, 20 and 22 gauges at least. The spools are a few dollars for several hundred feet. When I built my layout 40 years ago I got a few hundred feet of scrapped industrial 14 ga, instrument control wire, four strands wrapped as cable. This was from the destruction of a plant that had an explosion. They will not use recovered wiring during reconstruction.
For my train layout, postwar layout with ac wiring, I learned not to use paired or twisted wire on my Marx switches. If I ran the ground and 14 ac paired or twisted, induction between the wires occurred and the switches would not operate. After finding this issue I split up the 18ga lamp zip wire I used for things like lighting too.
Charlie