I have just gone through the process of getting seats for my 128 sedan. I had the original seats for the car and they are not confortable. I replaced them since the material was ripped and the foam was old.
There are many seats that fit in the 128 with modifications to the rails and making a seat bracket. You need to look at the height of the seat so that it doesn't touch the roof.
124 spyder seats will fit with mods.
Honda Prelude
Acura Inetgra and RSX
Volkswagen golf and jetta. depending on the year
I personally have put Honda prelude (89-91 year) seats with mods to the rail and making a bottom seat bracket to fit the Honda rails. Acura Integra (91-93 year) seats also fit and they are much more confortable then the original fiat seats. I would recommend Honda or Acura seats, nice bolstering and material.
As mentioned before Acura RSX seats also fit, they have the intergrated headrest with good bolstering.
I looked at Miata seats they also fit, but could not find a set that were priced right or in decent condition, you may have better luck.
I have attached a pic showing the seat and the bracket that I made to fit the seat.