The aftermarket world is huge. What kind of parts are you talking about? Audio...visit any local stereo shop or Crutchfield. Suspension/4x4 parts (the cool stuff IMHO

) search around and you'll find lots of sites.
www.4x4parts.com is probably a good one to start with and see what's available. Superlift makes some pieces for our trucks and so does some other companies. Just do some searching and see what you like. Many times, online houses (like JCWhitney, Summit Racing, Jegs) will resell the parts much cheaper than the actual manufacturers will.
Good luck on your search and welcome to the board.