Is there any place I could buy a 'veterinary exclusive pet food'????

My vet put my cat on special food, because she has allergies.

It is called Hill's Prescription Diet Feline z/d Low Allergen.

I would just go to the vet and get it, but I was wondering if there was anywhere online where I could purchase it for cheaper! It cost $20 for a 4lb bag!a from the vet!r