You can find these very useful accessories in fireplace specialty stores or even some hardware stores and big box home centers as well. A search done on your favorite search engine will provide information on where you can purchase them as well. You can even buy them online but keep in mind that you will pay shipping and handling charges as well.
In the end the fireplace is installed by a stone mason, who will connect the carved piece or pieces directly to the wall with specialized galvanized wall ties. The shaped stone pieces are usually connected together with bronze pins, the gaps filled in with mortar, and finally sealed with grout material. The whole process can take many days to complete, but the result is a magnificent custom modern fireplace with a distinctive style.