I was going to replace the seats in my 1994 LS and found what looked a good clean set in the same color on Ebay from a 98 model year, however when I saw them for real they were in a worse state than my originals with a tear and serious wear on the drivers side (US passenger side) so I left them.
If you are serious about it, as all the other members suggest get the originals recovered because you will be lucky to find a decent set from a donor car.
Apparently some have fitted LS430 seats into the LS400 but it does require some technical fabrication skills to adapt them.