I have Bayliss autovents on my greenhouse, got them as a special offer at GWL one year. I have also got an autovent for the louvres, but have never gotten around to fitting it....think I got that off eBay, might be worth a look.
I did have trouble with one of mine popping off in hot weather when it's fully open, OH superglued it underneath the clamps in the end..... :-\