Feels like this needs a refresh since it's been 2 years since a post. Lots of stuff has changed since then. Not sure if anyone needs this since NR2003 is pretty much dead here but it's worth a shot.
www.nnracing.com - One of the go-to places for NR2003. Bases, paint schemes, templates, logos, numbers, and pretty much anything else.
www.simracingdesign.com - Just like NNR, much more user friendly since you don't have people breathing down your neck and shooting you in the foot if you forget to credit a site for one associate logo.
www.outlawgrafxs.com - VERY high quality paint schemes and bases.
www.teamsbr.com - Older bases, logos and numbers. Abuchl is currently making 2015 paint schemes at 1024 resolution, making for a better choice if you have a lower-end PC.
www.4widedesigns.com - VERY high quality paint schemes. Batches of 60 or so highly accurate paint schemes (accurate grille tape, associates, side skirts, etc) are released every month.
www.bigevilracing.com - Templates for the new mods (NXS15, Gen6, CWS15), VERY high res logos and numbers, a great source for creating own content.
www.racingrafix.com - Just like BER, but with paint schemes and more frequent logos and numbers being released. Another great site, since their main logo creator Sage will help you if you need help with logo creation.
www.fse.marleyman.co.uk - Up-to-date tracks, mipdates and other resources.
racing.armorydigital.net - AMAZING fictional carsets, also the home of the UIE 2013 mod which includes faux tickers and replay skins.
rdracinggraphics.weebly.com - Logos, numbers, cars and bases. I am a staff member here. Forum Link
www.stunodracing.net - Render scenes, bases, and cars. Basically a reincarnation of the former Des Mods Racing site.
www.masgrafx.com - Logos, numbers, bases, cars, etc. WARNING - MUSIC VIDEO WITH MUSE AUTO-PLAYS ON THE SITE!
Above all, AVOID SIMDETAILGRAFX / NASCARDESIGNGRAFX!!!!
The site's owner, Craig / John is a damn liar who will read PMs of members and ban them out of paranoia or the fact that he doesn't like one of their cars!
See more in this thread: http://www.simracingdesign.com/threads/simdetailgrafx.56360/