There's the single disk honda mp3 player http://www.handaaccessories.com/accelect03.html $372 I've seen a bunch of theories that the 2003 accord uses an apline M-bus deck so in theory any M-bus changer would work, but I don't know for sure because I can't find anyone who has done it.

Or you can add one of these http://www.logjamelectronics.com/sounsnhond3.html and get a sony CDX 757 that's the only successful combo I know of for an mp3 changer to a 2003 accord.