I don't know if it's of much help, but I'm still doing the following:
In the distant past, I tried my hand at model railroads. Even then I was more into the scenery and modeling part than the technical bit, so even though there was no room in the house for a permanent layout, I did occasionally buy some small buildings. mostly Alpine cottages etc. The brand was Pola I believe, and the scale was HO.
One of these was never built and the parts ended up in my pile of random bits. And this small cottage happened to have a lot of windows with exterior shutters, which turned out to be perfect as small vents, considering they were louvered.
I've used them on vehicles, mecha, buildings and more, and they are just excellent. My original stock is becoming low, so I've switched to using oyumaru to make a simple press mould from them and making greenstuff copies whenever I need more. (The original plastic was a bright green to begin with, so even unpainted, they appear original
)
I suppose you could do the same; I'm sure there are more to be found in other scales as well.
On the other hand, I've also used Gunpla/mecha/Gundam sprues with circular turbine/fan bits in combination with some tubing to create airconditioning parts.
Then there is the use of screen/mesh as a grille, together with some plasticard to create something that looks like a vent, or making louvered bits with simple strips of plasticard, with some sort of frame/border around it.
And all of the above can then be moulded and cast in either greenstuff, resin or even plaster if you need a lot of them...
