Looking through molding resources for a current historical project I am bidding I found a few resources I thought would be useful to post here.
First, Hyde Park Moldings, has a variety of useful moldings. They also have an article describing the different materials molding is made from.
Balmer also has a variety of moldings available, as well as custom and made to order pieces. They also have pricing available online and cad drawings.
Focal Point also has some pieces available online, but I think the most interesting thing about Focal Point is their Quick Clips. This item is a faster system where brackets are installed to the walls, and then the cornice snaps into the previously installed brackets. They are also reusable if adhesive is not used, which means that they could be used for theatre work.
Since we are on the topic of molding anyway, Lee Valley has a tool that is also useful for mounting molding. In this case its a jig that helps you keep the cornice at teh correct height / angle, and then slipps out from behind the piece so you can move on to the next piece.