Hi tablesalt,
Don't know how much you want to spend, or the type of coving you want, but if you do a search for "Decorative Plaster Mouldings", you might get some pointers.
There are two common types of coving ........ the type you mention and Architectural (S type). Are you sure you don't mean CORNICE? This decorative moulding can be sourced in specialist workshops. Have a gander in Yellow Pages under Plaster Moulding. Or GOOGLE