Pat Wilson's Thunderbird Parts is the only known supplier of tonneau covers for 1958-1966 Tbirds. Presently, he is not making any 1958-1960 tonneau covers, but will if he receives 2 or more orders. $799 is the cost. Here is a link to them.
All Tonneau Covers include mounting hardware, instructions, and a black, white, or red headrest. The tonneau covers come in a white gel coat and must be prepped and painted to match you car. Tonneau covers are large and must be shipped by trucking company. They come in a crate, the dimensions are 64x44x24, and they weigh between 70 and 85 pounds.
Pat Wilson's Thunderbird Parts is located in New Jersey - http://www.wilsontbird.com