www.needswings.com = Engine, suspension, exhaust, some brakes, other misc items

www.ctd-germany.com = Exterior dress-up, if still in production.

www.vmaxx.de = More overpriced (and likely obsolete) dress-up goodies

www.pogiparts.de = Interior brightwork

www.chrometec.de = Random overpriced rubbish

www.ebay.com = occasionally you'll find some random NOS goodies on there, avoid everything else.

That's about it. Lots of research is the key, as each specific mod has had numerous iterations and revisions, some more effective than others but ultimately it's a taste issue.