A brand new stereo will come with the headunit side of the harness, and you can buy the manufacturer one from most stores (Future Shop, Best buy, Visions), and then just match the colours, and then you have a plug and play harness. As long as the OEM wiring was never spliced into by some shoddy install that is.
I usually grab the 2 harnesses, my crimper (~$10-$15 from can tire), some crimps (~$5), sit down for 5 minutes and make it myself. The only thing you'd really need to attach directly to the harness would be a remote wire for the amp.