I looked around on E-bay and have found all kinds of parts for my car. Carbon Fiber parts hoods bumpers, just about anything you may want and a lot you never even new where out there! I just typed in 98 mustang and got 1155 items.
There is a company called A.M.P. which stands for All Mustang Performance which has some nice performance products. There are TONS of Mustang companies out there for ALL kinds of different Mustang accessories. If you are looking for anything specific you could look in the magazine 5.0 or Muscle Mustangs and Fast Fords. I guess it all depends on what types of things you are looking for.
their is a world of after market parts for mustangs. try Year One, Summit, Jegs, the paddock, even JC Whitney sales mustang stuff. pick up any mustang mag. at a news stand,look threw the adds. that may be the best place for you to start