You can find the sheet music for the "Sanctus" from Missa Laetare

: While most Lenten music is somber and unaccompanied, the Missa Laetare often includes more melodic movement and, in many traditions, organ accompaniment to mark the midpoint of Lent.

: One of the largest online libraries of free sheet music. You can search for "Missa Laetare Sanctus" or the specific composer's name if known.