If you're going to carve a seat, you'll need greter than 12" wide. so be aware of this. If you glue pieces together, It doesn't carve smoothly over the joint since the glue is harder.
For back band and thigh braces you can also get Ethifoam. At discount stores (K-Mart) etc in the camping area you can find sleeping pads made of Ethifoam for about $7. Great for padding and back bands. I know people who have used those long floatable tubes (snakes) from toy stores for thigh braces. Those are Ethifoam. I purchased closed cell foam 1" thick at garden centers to kneel on.
Also don't forget your local kayak shop. Sometimes the slight price increase outweighs the shipping.