For animal patients that have been examined within the last year, it is legal to fax an original hard copy prescription to a pharmacy as long as it is written on an official New York State prescription form and is manually signed by the prescriber. Without a recent exam at the vet's office, however -- and in particular, without a blood test to make sure your pet isn't heartworm-positive -- the veterinarian may not be willing to issue a prescription for heartworm medication.
Professional practice guidelines at http://www.op.nysed.gov/prof/vetmed/vetpg5.htm#five11 say that veterinarians should only prescribe, deliver, or have delivered prescription drugs when a veterinarian-client-patient-relationship (VCPR) is established and the veterinarian has determined that the prescription drug is therapeutically indicated for the health and/or well being of the animal. Further, a VCPR can only exist when a "veterinarian has sufficient knowledge of the patient to initiate at least a general or preliminary diagnosis of the medical condition of the patient." The vet should not prescribe medication that could harm the animal or cause an adverse reaction, especially without a timely exam and without your signature on a form that relieves the vet of liability if you choose not to have your pet's blood tested.