Yes the diffuser is still available and it is in near new condition. It was on the car for a few thousand miles. It came off an 05 M3 so I see no reason it wouldn't fit an 04.
Current: 2005 E46 ///M3 - SG | CIN, 1999 E36 ///M3/2/5 - TiAg | BLK, 1998 E36 ///M3/4/5 - Estoril | BLK
Previous: 2002 E39 ///M5 - TiAg | BLK, 2010 E92 ///M3 - LMB | BLK
, 2008 E90 ///M3 - IB | BLK , 2006 E46 ///M3 - IR | CIN | ZCP | 6MT, 2007 E90 335i