http://www.nr2k3tracks.com/
http://www.theuspits.com/
http://www.masgrafx.com/masgrafxracing/
http://www.racelinecentral.com/NR2003_mods.html
The must haves.....
http://www.dwarehouse.com/ - Dave Hoffman's spotter sound pack
http://superspeedway.net/jla/ - anything and everything from jjohnson2004 is a must have for the oval racing.
I can't remember where you would find Methyl Ethyl's sounds, but he is/was the sound god of NR2003. Google search or search at either msgrafx.com or superspeedway.net for him. Make sure you google search "Methyl Ethyl NR2003" or you will just get mostly methyl ethyl ketone, which is basically a chemical solvent. Don't think that will help you out with NR2003