Actually aftermarket parts for the big rigs is only lacking here in the USA. In Europe truck modeling is quite popular. R&C Trucks closed up shop many years ago when the one partner passed awy. It then changed it's name to Clint Freeman Components. Clint closed up shop a while ago due to the fact that others were repopping his parts and selling them on ebay and such. His parts which were high quality so up on ebay from time to time. They are well worth there cost. The was another guy out there Bobby Griffen, he closed up shop because he was overwelmed with orders and could not keep up with the demand. Whether he reopened I do not know. Do a web search for Spaulding Trading and Shipping, located in Wisconsin they carry a good deal of aftermarket from all over the world. Kit Form Services aka KFS is another good aftermarket vendor based in England. Google there name for there web address. Howard offers quality parts (I've spent 100's with him) and the parts usually shop here in the states with in days of the order being placed. Another US based one was Mo'luminum. Not sure if Jamie is still producing parts or not. But he made some very nice turned and machined aluminum parts. These are just some of the aftermarket guys I remember from my 10+ years of building big rigs. You can also check the big rig specific truck modeling forums on the net also as there are other models out there who dabble in the big rig aftermarket and sell with in these forums only. Also watch the model truck section of ebay. I have purchased parts and decals from people there as well as that is there only web presence.
http://www.stsmodeltrucks.com/
www.kitformservices.com
http://www.timstrucks.com/
pete389.proboards.com/index.cgi
www.auslowe.com.au/index.php
Hope this helps