I know bobroberts sells just about any color you'd want. I tend to re-use old jamma harnesses and any wires i've de-pinned. I'd love to know of a one stop shop for absolutely any color combination tho.
Harbor freight has cheap wire from time to time.
If you looking for something special these people will have it. http://www.anaheimwire.com/about_us.php
Orvacs, ford eltronics or jk eletronics have wire.
Cheapest wires come from the TRW or ACP swaps or any other ham swapmeets.
They don't have the best prices and only 5 colors (red, green, yellow, black and white), but I get a lot of my wire from Fry's. Also from a local Houston place, EPO, they have more colors but are farther away.
I buy my wire from a local car audio shop. The owner orders me what I ask for when he orders his. Its not the best stuff around, but it get the job done.
$20 shipped to anywhere in the US. Note that the wire meets spec 2464 (was slightly misrepresented in the original sale) - good for carrying signals, but not so great for lots of amperage. Since you are wiring a control panel, it would work just fine. I didn't have any complaints about it from anyone on this forum during my sale (18 happy customers).