San Antonio Public Libraries do not offer exam proctoring. However, here are several places who have offered proctors in the past. You'll need to inquire about proctoring fees.
Counseling & Testing Center at St. Mary's University;
San Antonio College Assessment Center (proctoring of exams only for current SAC students as of 8/28/13)
Palo Alto College
UTSA Testing Services
Our Lady of the Lake University