Check out the exploded views of the truck that came with it or look at the Traxxas website you can get them from there. With the exploded views you can see how the parts go together and the part numbers are right there as well so you know what to get if you need something.
Once you know the part number then you can check ebay, Tower, rcplanet, etc. for what you need.
These are links to the exploded views for the 4907, the same as the 4908 except the 4907 came stock with an FM TX/RX. The chassis, drivetrain, and suspension etc.etc. are the same: