Yes, San Antonio actually has a very healthy rental market due to the presence of the military. Fort Sam Houston alone has over 50,000 soldiers stationed there. Randolph AFB has at least another 20,000. This creates a lot of opportunities for an investor to buy a home for rent. There is also a large population of students in the northwestern side of town.
To target military families, look for homes around Schertz or Cibolo. For students, look for newer construction homes around DeZavala, Borne Stage Rd, or NW Loop 1604. I do not work the San Antonio market but used to live there for several years. If you would like a referral for an agent specializing in investment properties in San Antonio, please let me know and I will try to find one for you.