You are looking for "fixture wire". That might help your search. My electrical supplier sells it -- by the roll. At one time, I always needed it and didn't have enough. Eventually I scraped up enough here and there that I have enough for occasional projects, so I've never bought it by the roll.
Just searched for it on the Web and found
this wire for $15.54 for 100ft. plus shipping. I didn't try to find a better price, but that is a good starting point.
In a pinch, I've used 12ga. THHN building wire and screw-terminal sockets, but that's much harder to work with.