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Leroy13 said..Don't Panic!

I used a bit of BluTac for 6 months and/or some Gaffer tape for two months and then I found the vent plug where I put it. Unless you are paddling fully submerged you won't get any water in your board.
That is a very dangerous advice: if the water is cooler than the air, putting the board in the water will create a depression that will suck through the vent with great strength, most probably bypassing the tape.
I would try windsurfing shops, their plug screw should be of the same dims.
Otherwise, I would just resin/glue a patch on top of the screw hole, protecting the hole hiself via masking tape.