Here is some good resources:
Stock cars: http://www.netcarshow.com/
Body Kits: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=21
Rims: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=23
Doors: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=14
Sound System: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=16
Interior: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=20
Intercoolers: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=13
Engine Bays: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=18
Exhausts: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=19
Air Filters: http://z4.invisionfree.com/CSC_Resource_Area/index.php?showtopic=17
Carbon Bonet: http://thumbsnap.com/sddM3Ksj?src=tsd
Good for kits (Click on the car model in the navigation): http://www.aeromaster.jp/index.php?cg=2
Number Plates: http://www.licenseplates.tv/european-plates-2.html
More Wheels: http://www.tuningfever.fr/fr/pics/cat-90-wheels-jantes/page-2.php
Vinyls: http://www.innovativevinyldesign.com/VehicleGraphics.html
Decals: https://2img.net/r/ihimizer/f/decalsheetbiggergf9.jpg/
And many tuned cars!: http://forum.4tuning.ro/158-tutoriale-and-help/58268-donors-poze-ajutatoare.html