if its just a one off then find a decent panel beater to fabricate a new one from scratch, not something your average panel beater can do i know but try the hot rod guys, they do stuff like that all the time. one of the guys over here (NZ) has had his door sills and most of his floor replaced and they put the factory mounting points and all the dips and rises in, it looks like factory.
besides that the only people i've heard of that manufacture any replacement bits for early celica's is Toyhead (www.toyheadauto.com) in America, and from memory they only do the door sills.