Grady White still makes the 306 Canyon to this day, of which if the seat looks the same or is similar to the new ones, it's possible that it might be interchangeable. A quick call to your local GW dealer might resolve the issue.
Best to contact GW Cust Service or your dealer. The seat is designed by GW and I believe is manufactured by Llebroc currently. GW owns the design and has moved too various manufacturers over the years. The manufactures cannot sell directly to customers. I suspect it will be expensive.