Moldings and decorative elements can be hard to find and specific to the manufacturer. Below are a variety of resources to help locate decorative elements, or can assist with ideas when designing.
Frank Morrow Company has a variety of metal stampings and architectural details.
Decorators Supply has a large variety of wooden and plaster decorative details. While you can order their catalogs (which are great as a resource), you can also browse online and get a variety of CAD downloads.
Cast Design Supply
Van Dykes Restorers
Historical Arts and Castings