You probably don't see a lot of aftermarket seats, because of the cramped interior in the Fiero. The stock seats are pretty small. And you can't go much bigger than that.
That said, I was able to fit Corbeau Forza seats in my Fiero... but just barely. They are 20" wide at the hips, and 23" at the shoulders. The shoulder bolsters were almost touching the door panels.
The seat height was also a concern. With the sliders and Corbeau seat brackets, the seats almost touched the headliner. So I made custom seat brackets, to bring them down.
I don't have many photos. The ones I do have are old and low-res. (I don't have the Fiero anymore)