Here is a letter written by a fellow breastfeeding mom (Thank you for sharing this Kerry!). Her pediatrician gladly signed it and said it was well-written. We both expect it to do the trick. If someone else needs such a letter, feel free to use this one as a start:
To Whom It May Concern:
The American Academy of Pediatrics supports the medical benefits of breastfeeding in its position statement, which notes that, “Breastfeeding and human milk are the normative standards for infant feeding and nutrition. Given the documented short- and long-term medical and neurodevelopmental advantages of breastfeeding, infant nutrition should be considered a public health issue and not only a lifestyle choice .” Physical separation of mother and baby presents a medical need for expressed breastmilk.
[MOM’s NAME] and her [SON/DAUGHTER] are separated for approximately 10 hours a day 5 days a week and occasionally for extended periods of approximately 2 weeks. Breastmilk accounts for greater than 50% of her [SON/DAUGHTER]’s caloric intake and there is no comparable substitute for the immunological properties her breastmilk provides. During these periods of separation, a breastpump is the only means for providing breastmilk to her [SON/DAUGHTER]. Therefore, [MOM’s NAME] requires a quality double-electric breastpump (such as the Hygeia EnJoye), any necessary replacement parts, and/or a quality manual pump.
In addition to providing optimal nourishment and antibodies for [MOM’s NAME]’s [SON/DAUGHTER] during periods of separation, using a breastpump addresses the following medical conditions for [MOM’s NAME]: (1) prevents engorgement of the breasts (ICD-9 code 676.24); (2) reduces the risk of plugged milk ducts and treats plugged milk ducts should they occur (ICD-9 code 675.24); (3) reduces the risk of mastitis (ICD-9 code 675.14 or 675.24); (4) maintains milk supply/prevents suppressed lactation (ICD-9 code 676.54).
 American Academy of Pediatrics Section on Breastfeeding. “Breastfeeding and the use of human milk.” Pediatrics, vol. 129, no. 3 [March 1, 2012]:pp. e827-e841.