You can get a Momo RS GT for under $300 ea. here:
R&W Performance
Note that the RS GT and RS2 GT don't have tilting seat backs. The back angle is adjustable with the rotary knob, but you can't flip a lever and tilt it forward like your current Porsche seats. To get that feature, you would need to get the Street Racer models or something like a Recaro SRD.