I remember reading something that said Jack Miller (in Kansas City) was a good source now. They are a "plaza" of multiple dealers like Kia/Subaru/etc... Googling their number gets: 888-918-6252. I can vouch that they've been around forever in KC...since I live here. I also talked to their service guy a couple times over the years and he is knowledgeable. Haven't talked to the parts guy(s) though.
IsuzuParts.com shows the thermostat "neck" for $108....here. That implies it might still be available to order...if you can't source a used one.
Hope that helps.
Gregg
2001 Ebony VX and 1989 Custom 383 Corvette