You can select San Antonio area from the drop down in the HAR home search for some listings which pull into the HAR MLS. If the MLS partners with HAR you may see the listings there.
Unless you know the San Antonio market well and are in San Antonio frequently, it may make more sense to refer to a San Antonio agent that is a market expert. You could still get a referral fee and be involved.
You will need to join the SABOR MLS to have agent level access to their listings. On their website there are instructions on how to join the MLS Only. Here's the link, https://sanantonioboardofrealtors.formstack.com/forms/mls_only_agent