I got the 993 RS engine mounts and the appropriate nuts & bolts to fit (about £150), there is a thread about it if you search for 'engine mounts'.
I'm not saying that they are a 100% guaranteed cure but they along with the gearbox mount seem to have cured mine which might seem strange as stiffer mounts usually add to NVH, that said these are not the race version mounts.
Try adjusting the seat rake first and identify what is causing the vibration at idle, if it's happening at speed it could be the tracking etccausing a vibration which in turn starts the seat off.
I'm not saying that they are a 100% guaranteed cure but they along with the gearbox mount seem to have cured mine which might seem strange as stiffer mounts usually add to NVH, that said these are not the race version mounts.
Try adjusting the seat rake first and identify what is causing the vibration at idle, if it's happening at speed it could be the tracking etccausing a vibration which in turn starts the seat off.