Vent O-ring

[tmsleung]

I stopped windsurfing long before this things appear on boards. Stepped on one of these things this season and I got suck in again, just got my first used N-ASA Formula board. Unscrew the vent and the rubber ring is in bad shape. Questions:

1) Where do I got these rings, can I use those from hardward stores?
2) Do I need take the screw off everytime? If not, would it be better to seal the threads with marine silicon?

Hope you can help. Thanks.

Simon