My best luck has been buying shortish bits of multi conductor wire at electronic surplus shops.
There are 10 US standard wire colors (black, brown, red, orange, yellow, green, blue, violet, gray, white). Once a multi cable - each wire must be identifiable - has more than 10 conductors they tend to use colors with stripes or dashes of another color. Sometimes they throw in non-standard colors (ie pink, light blue, light green, etc). If you can find a 25 conductor 16ga cable, it will have 25 different wire color patterns. And - these cables usually have the high flex wire that we want for a harness. I've found 100+ conductor cables where they use multiple color dashes, but only in 20ga or less. I keep looking, though...
Note that some multi cables have "twisted pairs" - half the conductors are just white, each white one twisted with a color coded wire. Humbug.
