Whenever I need a technical or mechanical diagram, I always check Google Images first. This time, looking for a diagram of a timing belt for a Toyota Celica 1991 with a 4AFE engine brought up generic Toyota diagrams such as this one:
If this isn't the right diagram for a 1991 Toyota Celica, there are plenty of other diagrams to check. There are other more specialist stock photo libraries you can try, although these may charge a fee for their services.
Elsewhere, morediagram.com/docs offers downloadable PDFs of vintage Toyota timing belts, which may include the 1991 Celica, while aa1car.com/library not only carries Toyota timing belt diagrams but also advice on when the belts should be changed and sources for parts. 2carpros.com offer a similar service.
Why Not Join the Toyota Community?
There are online communities and forums for just about every make of car imaginable, and Toyota is no exception. You could join toyotanation.com, post a question on timing belt diagrams, and see if other Toyota drivers can help you out.
YouTube is a good place to look for general tips on car repair. This film shows how to replace a timing belt on a Toyota Celica/Camry.