I’m thinking of flying and want to take my emotional support animal “Daisy” with me. 

Assistance Animals
Trained assistance animals traveling with and providing assistance to a Customer with a disability are accepted in the aircraft cabin.
Trained assistance animals and emotional support animals will be allowed to travel on flights to/from all domestic and international destinations. Please note trained assistance and emotional support animals may be subject to additional governmental laws and regulations at the destination, including but not limited to, health certificates, permits and vaccinations required by the country, state or territory from and/or to which the animal is being transported.

Southwest Airlines

I found an example of an acceptable letter online. I figure a buck and a half, a jug of Ripple wine and two marijuana joints as payment ought to ensure that a competent professional will sign.

SAMPLE LETTER

(on professional’s office letterhead)
DATE

TO WHOM IT MAY CONCERN:

(Patient’s name) is currently under my professional care for treatment for a mental illness defined by the DSM-IV. His/her mental impairment substantially limits one or more major life activities. I have prescribed an emotional support animal as part of the treatment program developed for (patient’s first name). The presence of this emotional support animal is necessary for (patient’s first name)’s mental health.

I am licensed by the state of (state) to practice (medicine/psychiatry/therapy–choose applicable). My license number is (license number).

Please allow (patient’s full name) to be accompanied by his/her emotional support animal in the cabin of the aircraft, in accordance with the Air Carrier Access Act (49 U.S.C. 41705 and 14 C.F.R. 382).

Sincerely,
(doctor’s name and title)