Here is a link to the new Posada database posted by Dr. Ricarda Musser at the Ibero-American Institute – Prussian Cultural Heritage Foundation in Berlin: http://digital.iai.spk-berlin.de/viewer/collections/joseposada/. The collection includes hojas volantes and folletos.
This is helpful to us since the Graphic Arts Collection only holds one print by Posada:
Jose Guadalupe Posada (1852-1913), Calaveras zalameras de las coquetas meseras (Mexico: Por la Testamentaria de A. Vanegas Arroyo, no date (ca. 1917)). Graphic Arts Collection GA2007.03749
The Princeton University Art Museum has several works attributed to Posada: http://artmuseum.princeton.edu/collections