Waiting for the staff I can only tell you that as far as I now they are standard for almost all windsurf board, so you can check on the nearest surfshop. I can say it because my 2009-2010 Starboard Serenity is fitted with a standard vent screw, the same provided on others board of my quiver.
This topic gives me the possibility to ask another question. Is there a reason for the common use of that imho weird and bulky type of screw? Why not using a simple M6 screw like the one(s) of the fin, screwed in a serious m6 steeel insert?
On my former F2 Wizzard I waisted almost immediatly the original vent, so I fixed with epoxy an M6 insert, like the ones widely used in snowboard industry, cheap, strong and light, and it lasted perfectly for many years of use since I've sold the board. After that, for various reason, I've replaced the original screw on a lot of boards, easily on my own, in few minutes of home work.
Consider that a steel M6 can be found almost in every wharehouse of the world, instead of rare dedicated (M10?) windsurf air vent screw, so in case of trips or living far from a surfshop (nowaday unfortunately it often happens) one can find this critical spare part very easily.