I have found stripping multi-conductor cables a good resource for wire. They are stranded (which I prefer for flexibility), and they are color coded, or sometimes numbered.
One source is old printer cables (the db25 pin cables).
It can be a pain in the asbestos to strip the outer jacket sometimes, but the prize inside is worth it.
I have also bought spools of wire off the net and even Radio Shack, however Radio Shank tends to be grossly over priced, but it works when Im in a pinch and a hurry, since I have a store locally.