Truck Shop in Orange, CA.
4-5-6 Truck Parts might have them.
However I don't think it came with one, but it had a donut looking unit that was a seal under the floor -
http://67-72chevytrucks.com/vboard/s...4&postcount=22
Assembly manual picture -
http://67-72chevytrucks.com/vboard/s...8&postcount=10
And pics of the original install -
http://67-72chevytrucks.com/vboard/s...6&postcount=29