From 1919 to 1937 the mental health efforts of the Commonwealth of Massachusetts were run by the Department of Mental Diseases. The massive bureaucracy needed to care for the insane, epileptics, and the "feeble minded" was truly stunning in it's scope. The DMD supported over a dozen mental hospitals and state "schools", spending up to one sixth of the Commonwealth's entire annual budget on direct care, facility maintenance, and administration.
Anything involving an involuntary committal, an incident of violence, criminal behavior, or the unusual would likely result in some correspondence from the Department being part of a patient's file at Arkham Sanitarium. Those conditions cover the circumstances of almost every mention of the Sanitarium in Lovecraft's work, so this letterhead should come in handy.
Click here and you can download the PDF from Google Documents. Open it in a forms capable reader and you'll find text fields for the date, address, and body text with an embedded typewriter font (the freeware "Veteran Typewriter" by Koczman Bálint). Just fill out the fields and print.