Hi Trakka,
I've just being thru this myself.
The only aftermarket suspension seat that fits these cabs, afaik, is the Stratos, also known as KAB to some suppliers (KAB own Stratos).
The part number for the front passenger seat is 904LH (LH just means the adjustment knob is on the LHS but either would fit these cabs, just in case you find a 2nd hand one), these are the same as that get fitted to landcruisers.
When I bought my truck 2 years ago Stratos did not have a fitting kit for the passenger side, ATW have their own kit, as I wasn't sure if I wanted to change the seat I bawked at the $2800 all fitted/certified price from ATW and just bought their fitting kit (2 rails and bolts/plates) approx $140.
I bought a new seat 2 months ago thru TJM for $1940 but couldn't get the ATW mounts to fit, turns out I needed to order the slide rails as well from Stratos, the mount rails bolt to these, another $280. At this time Stratos queried TJM why I only wanted the slide rails as they now offer a full fitting kit for the passenger side of an Isuzu. i didn't bother asking the price as already had my other ones.
My engineer is happy with the ATW set up.
I have a full set of photos from ATW which show the mounting points if your interested, just PM me your email.
In hindsight I should have just got ATW to do the lot.
Cheers Steve