modifrys harness is the same as any regular stereo harness, it just happens to have extra wires for the modifry DCI so that you don't have to splice into the factory wires. It's only needed if you are getting the DCI, if your not getting the DCI any aftermarket harness for honda will work fine. He has a plug and play which is already pre-wired and spliced for you, and one where you will have to do the splicing and wiring yourself.

 

Here's Modifry's page about the harness:
http://www.modifry.com/products/dci/adapter.htm
Most of the generic adapter harnesses (Metra and Scosche are two big
makers) were made for the Accord/Civic and don't have the S2000-specific
dash control & mute wires.
When I added my DCI, I had already installed my headunit with a generic
adapter harness, and I ended up moving some unused wires/pins to the
dash and mute places, which was a PITA so I definitely recommend using
the correct harness right from the start if you have the option.