LMC and Carter's carry them. They are similar but NOT identical to the factory mats. Close enough though.

http://www.lmctruck.com/icatalog/cbe/full.aspx?page=56

http://www.oldchevytrucks.biz/55_66/full.asp?page=39

PS. Don't forget the pad underneath. Better yet, save your original one if you can.