Well - after much lurking here and seeing all the tutorials on folding fabric - I just had to try to find a reasonable way to fold my fabric.
P.S. - they call it Corex on the invoice.
I went to a local company called Multi-Craft Plastics in Tigard, OR.
He sold me 2 4ft x 8 ft sheets of white for 12.60 each. Then he charged me only 2.00 more to cut it into 11 3/4 by 7 1/2 inch pieces. Rather - I have a paper cutter so I had him only cut the 11 3/4 inch long strips and then I will use my cutter to cut those strips down to the 7 1/2 inch wide.
I will get 48 boards out of each piece of sign board. I will see how this works and then get more for smaler pieces. But I figured that I need the larger pieces to be folded most. I will post a pic of the corrugated plastic in a minute