Vent Openers, thermally actuated.

[DavidCampen]

I am just finishing construction of my self designed and built greenhouse. I am using Bayliss MK7 vent openers:
http://www.baylissautovents.co.uk/shopexd.asp?id=172
Actually, I am using this model in the "orchid" variant that starts opening at a temperature about 10 deg.F higher than the standard model. The only place in the US that seems to sell these is Charley's Greenhouse:
http://www.charleysgreenhouse.com/35...ent-Opener.htm
I have purchased several of them from Charley's but they always seem to be on backorder.

I am a bit surprised that the Bayliss MK7 Orchid Vent Opener is not more widely available here in the U.S. What vent openers are other people using?