I've tried to keep quite, but there is one thing people should realize . . .
There are only a VERY few manufacturers of these things world-wide. The best come from exactly TWO companies, regardless of the name or logo on them.
Arguably the best of the two is Autovent Shades (AVS). Their own branded ones are the cheapest for quality there is. Many of the other branded ones are the exact same thing, but with other logos and higher prices for the privilege.
If the vents your looking for look practically exactly like the AVS ones, there is a VERY good chance that they ARE the same thing.
(I can't state my source, but it is reliable and I'd put my own money on it happily.)