I highly recommend any kits from Arbor Assays - here's the link for the cortisol kit (http://www.arborassays.com/product/k003-h-cortisol-eia-kit/?highlight=cortisol). The company is fantastic - they actually do validations for a variety of sample types and the technical support is just excellent. My lab has had great success with their kits using many different sample types from a variety of species.