If you only have to rebuild one, then you shouldn't worry that much, as there are a huge number of normal rebuild parts still available, and many others as well. If you want to build one from scratch, that is another story, however.
I am just completing a huge refurb/restoration project on my '89 SC, and I have not had that much problem replacing even out of the ordinary rebuild parts. That being said, however, I have found over the years that Toyota ocassionally gets their part numbers screwed up, such as when they claim a part is no longer available, but actually is, but is now only listed for the NA model, or perhaps only for earlier year models, but is actually the same part for both. You just need to have some patience and learn how to research ordering the parts. Additionally, there are still an awful lot of parts available online as well, and can be significantly cheaper than Toyota, even if you get a big discount like I do (I'm putting some of their grandkids through school
). A good parts counter person won't mind if you shrink from paying their sometimes outrageous prices on parts, and may even be able to source generic parts for you.