I am in the midst of building several ConCourse level HO slot cars. Anyone know of a good source for items like fender screens, radiators, gas caps, wheel inserts, etc?

To be honest, there really isn't a "source" for those type os small HO parts......you'll need to scratch build stuff like that.

The best thing I can say is use your imagination....... ;)

try the ho train magazine "Model Railroader". they have several tip books with all kinds of great ideas.