There's at least 3 or 4 differant styles of valves, hundreds of differant companys making them, valves do not use gromits, so I guess the ansewer is no.
If it's a gate or globe style valve it's very common for them not to shut completly off or leak due to there poor design and should be replaced with a ball valve.
any good hardware store should be able to help you, you can get a pack of assorted sizes of rubber washers. if by grommet you mean the packing, the hardware store can help you there also.
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