looking for a thermal switch for water cooled system

[Kroffty]

I am building a water cooler for my system and I need a thermal switch to turn on the radiator cooling fans around 90 deg.
I have looked at radioshacks web site and I cant find anything like this.
Does anyone have some suggestions on how to do this. It would really cut down on the fan noise if they dont run all the time.

 

[Smbu]

A Digital Doc 5 might do it, but I'm not sure so you should check out some reviews of it to see.

 

[Kroffty]

I looks like a good unit but a little over kill for my needs. I was hoping to spend less than $15 or build it my self for cheap!

 

[dkozloski]

Any electrical supply store will have thermostatic switches of all kinds.