Most or probably all all aftermarket seats require some type of special bracketry/sliders that mount the seats. Depending on what type of seat you're looking at you can get a non bucket style seat or reclinable bucket style. Some brands are Sparco, Recaro, Bride, TRD
etc. Then If you have the required brackets and pieces for the seat installation should be as simple as removing the stock seats.
The celica seats are bucket seats which is why I didn't get rid of mine. I have friends who have bucket seats they hold grip you in real well. Some are too snug tho. Depends on what you want to use them for. My friend is selling her Sparco's btw