You can buy 4 x 8 ft sheets of it at any plastic supply house, just ask for Coroplast. They will probably have white in stock since it is used for outdoor signs. 4mm is the common thickness, but I used 8mm for greater rigidity.
For the seams, use aluminum 'H' trim (used for plywood joints) from HD or Lowes plus a few screws thru the trim and Coroplast so the wind doesn't pull it loose.
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'11 Crossroads Cruiser 32MK loaded