Are they really getting out for good? I just figured they would keep selling re-badged pickup trucks to fleets as long as they could turn a buck or two.
The good thing is that they sold a lot of Troopers over the years, so rebuildable parts - alternators, starters, even engines should be available for some time, I would think.
The 4L30E transmission was widely used, most notably by GM and BMW, so parts should remain available here, too.
Because of the GM tie in with Isuzu, some parts used were interchangable with GM cars - I'm not sure how many, but at least EGR valves are (which is good, because they like to go bad).
I'd also imagine some dealers would keep their service departments open for the brand just because they are money-makers - and since Isuzu isn't out of the car business overseas (and sold a bunch of Troopers there, too), then factory parts would still be accessable - although they might take more time and be a bit more expensive. They still have their commercial dealers, too - so they might be able to source parts and do service.
Down the road, it might be a little more difficult, but I don't think it will be too tough to find parts for the life of the vehicle. Fortunately, they built our Troopers pretty solid, so hopefully the big things won't be necessary.