Hi, ednjenn. Check at your local home center. The last time that I was in one, I saw what were basically kits of parts to be combined into surrounds. They had parts of various dimensions to fit various sized fireplaces.
Another way is to design it yourself and go to a full service hardwood supplier or woodworking specialty shop. We have several in the area that will supply, cut and mold wood on order for whatever project you choose and help with the design. Please be aware that this will be very expensive but buying the tools and equipment to allow you to make yourown will be expensive also. And don't forget that you will need to make a couple of smaller projects to give you some experience with the tools and finishing.