pressure release valve

Can anyone tell me if the object in these pictures is a standard item available in hardware stores and what it is called?
http://www.geocities.com/t5gtev/circle.jpg
http://www.geocities.com/t5gtev/slit.jpg
It is a circle of rubber 1 1/16 inches in diameter, 1/16 inch thick, with a slit in the center 1/8 inch long . It is used as a pressure release valve in these products:
http://www.oztops.com.au/store/products/item8.inetstore http://ezcaps.juneau-alaska.us /
I am wondering if I can buy these at home depot or somewhere like that, and I'd like to know what they are called, what department to look in etc in case the person I ask in the store doesn't know.
Otherwise, does anyone know where I can buy a sheet of rubber 1/16 inch thick?
Thanks
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I've never seen them. Maybe a brew shop would have them?
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