Ebay is a good source for parts, especially since you're probably not too worried about "vintage correct".
I like to put small plastic parts in my pocket with my keys and change and walk around for a few days with them. This creates a nice natural scuffing. Larger plastic parts can be dusted up with a kitchen scrubbing pad.
Boiling hot strong black coffee and tea can be used to "age" plastic. I mix in some dirt and oil. Do not overdo - the coffee has to be stupid strong and insanely hot, but once the plastic starts to change color, it will darken very quickly.
Nickel parts can be aged with acid fumes. BE CAREFUL, as the acid itself is caustic, and it is easy to overdo this.