First look at Home Improvement stores and if that does not work I would try a heating and cooling store that specialize in carrying these types of parts.
A thermostatic fan switch can generally be purchased at any electrical supply store. You can also try Specialty supply stores like Grainger or the big box hardware stores like Home Depot or Lowes. These are fairly off the shelf components so you shouldn't have too much trouble finding them. If all else fails contact a local HVAC outfit.
Your local hardware store such as Ace, Home Depot, Lowe's, or whatever store that is in your vicinity should carry it or order the part. If not, you can call the manufacture and see what steps you should take next.
