I rented a Matthews 12x12' white silk for an event earlier this year. It was actually multiple pieces of fabric sewn together. Does any other manufacturer make a single sheet that is 10x10' or larger?
You probably wont be able to find 12' but I've used Warehouse Fabrics before and they were great. They carry a range of 9.8ft silks. http://warehousefabricsinc.com/RIGAP25.html
Search for 118" silk and it will bring up the other colors and pattern choices.
I just bought a 20x20 polysilk from http://www.canvasgrip.com/ <-- the price was right but it does have four seams on it, however they did not make themselves visible in the finish of the car I was shooting.
If I had the money I would get fabric from Dazian and have them make me a 20x20 with a single seam. I might be in the market for a 20x30 in 2011.