Rick at Toyhead auto has a collection of new/old stock -
www.toyheadauto.com
There is also a couple of businesses in Thailand who make reproduction parts like indicators, interior door handle surrounds, etc
They sell on Ebay.
There are also wreckers but classic Celicas are not very common any more.
In general, you are on your own.
The industry doesn't really support the early Celicas.
Luckily, most trim can be custom made without any drama.
Seats and doors are easy to retrim.
And carpets are available from almost all automotive carpet suppliers.
- Stepho