Make Replacement Seats
RandyA (Randy) remade some Honda Odessy seats and used them as replacements in his Volvo.
Also, many newer seats have removable covers. You may need a few hog rings and a tool to put them back on but that part is not difficult. If you can remove the cover, run it through the washing machine. Do not use a hot dryer - either drip dry or use low dryer heat.
Unfortunately, all of the bucket seats I salvaged for the Fiero based kit cars are too narrow for the HDT and do not have arm rests. IMHO, the HDT needs a 20" to 22" seat.
What I am saying is just because a good seat may be dirty does not mean it can't be re-purposed. The pressure washer and sun will work wonders. Just be careful with the pressure washer. Too much pressure and too close can actually cut the upholstery fabric.