How do the 240z's deal with that sort of stuff?
Not just the S30 chassis Z's but also the early Celica, Corolla, Corona. There exists reproduction door, window, and trunk seals for those cars.
http://toyheadauto.com/Celica_Parts_1971_to_1977.html#Weatherstrip_
you can see the support for the older Toyotas is there.
http://toyheadauto.com/Web_Site_Directory_Toyhead_Auto_Restoration_Services_and_Parts.html
There is just a huge void that has yet to be filled for the A70 Supra. I think it is for the most part due to owners not willing to pay for parts as you've expressed. Those owners will dwindle in numbers as they become disenchanted to continue working on the cars, leaving us with individuals that really want to keep these cars going.