Dr. Cupples was a prominent San Antonio citizen at the time of his death, so if I were you I'd start there. Trinity, St. Mary's, Incarnate Word, University of Texas at San Antonio, Our Lady of the Lake are the universities there. The local history museum is the Witte.

Trinity, at least, is offline for the summer, but it's still possible to call or visit them during their summer hours.
http://lib.trinity.edu/lib2/spec_coll.php