I have a StudioTech stand and they sell a lot of fan options for cabinets, including some that are thermally triggered. They are not the only vendor, but you can see a lot of the options via their web-site.

I didn't buy fans from them because I created my own thermally controlled fans the cheap way. You can buy the rack mount cut outs and mount 12VDC fans to them, using the 12VDC of your pre-amp to trigger a relay for the fans. Then, I use the old down and dirty mercury switches that were common to older homes, cause I had one lying around, inverted it and then wired it as the swith for the fans from the relay. I just rotate the mercury switch enclosure to change the turn on temp. I have a non-contact handheld thermometer from RayTek to help me spot check the temperature.