Where can I look up FAA Letters to Airmen?
They are searchable at FNS NOTAM search: https://notams.aim.faa.gov/notamSearch/ 

A link is provided to a PDF, containing a link to the actual LTA.

(Filter by LTA, remember the agreement might not be with the airport, but with the ATSU, like the local TRACON/ARTCC, try searching by airport name + radius).

Expiry date is shown in the details view.