I used it for my cat that had kidney failure. My understanding is that it's a prescription diet only available from vets, so I don't think you can get it online. If your cat's having kidney trouble, diet is very important, so you need to stick with the prescription stuff. However, Purina, Waltham, Eukanuba and others also make prescription kidney diets - I'm not sure about the price comparison but you can ask the vet.
This website www.felinecrf.com
has lots of information about kidney disease in cats, and this page www.felinecrf.com/managd.htm
covers various diets available.