Are you sure it is an RA28?
A N.American spec 1976 liftback would be an RA29 with a 2200 cc 20R engine.
The RA28 had a 2000 cc 18R engine and was not sold in N.America.
Anyway, trim parts are notoriously hard to find.
The car is 35 years old and there is practically no aftermarket industry for it.
But all is not lost.
Wrecks pop up from time to time on craigslist.com
http://www.toyheadauto.com/ has many original parts and some reproductions.
There are web forums such as
http://groups.yahoo.com/group/Classic-Celica/
And I have have heaps of manuals, brochures, parts catalogues and other info on my website:
http://members.iinet.net.au/~stepho
Feel free to ask any questions.
- Stepho (Australian spec 1977 RA28 with Japanese spec twincam)