Re: blower vents?
WM does carry them, but as far as finding a set that's your correct size is pretty much a 50/50 shot.
If you call them and give your dimensions they could tell you if they could be ordered.
Another thing to consider - if you plan on keeping your boat for awhile - would be to have some made out of stainless. No need to worry about them cracking. It goes without saying that you're gonna' pay a few extra boat bucks for them.........................................

If you want to post the size you need ( interior flange measurement ) I'd be glad to poke around in this neck of the woods for ya'.