You can actually look this up in Google Display Planner. Log into your Adwords account and go to Tools and Analysis > Display Planner. Select the topic for World Localities>San Antonio under the Individual targeting ideas tab. "Texas > San Antonio" should show up in the box in the upper right. Now, select The Placements subtab, and Sites. this will give you a list of sites Google thinks is relevant to this topic. The top ten are <a class="sMDB" title="sanantonio.com">sanantonio.com, </a><a class="sMDB" title="kens5.com (MojoPages.com, Multiple locations)">kens5.com, </a><a class="sMDB" title="beaumontenterprise.com">beaumontenterprise.com, </a><a class="sMDB" title="thesanantonioriverwalk.com">thesanantonioriverwalk.com, </a><a class="sMDB" title="tubetexas.com">tubetexas.com, </a><a class="sMDB" title="mywesttexas.com">mywesttexas.com, </a><a class="sMDB" title="riverwalkguide.com">riverwalkguide.com, </a><a class="sMDB" title="hillcountrycurrent.com">hillcountrycurrent.com, </a><a class="sMDB" title="zacktravel.com">zacktravel.com, </a><a class="sMDB" title="corsicanadailysun.com">corsicanadailysun.com.</a>
Of all Google's targeting options, I've had the least success with Topics. I'd recommend picking sites from the list that appears, and target them directly via Placements, or pages within those sites if they're not all about San Antonio. Topic targeting is specific to every page, so if you use it, your ads won't appear across all the domains above, but only on pages deemed by Google to be relevant to this topic. I just think it doesn't work that well yet. I've used it with success with other targeting options like interest categories. For example, you could pick an interest category for Travel, and then combine it with this topic to hit travelers reading about San Antonio. Pretty cool, right?
Also, be aware that the default location settings for Adwords will show ads to people around the world looking for San Antonio, as opposed to people specifically located in San Antonio. If that's important to you, make sure to expand the Advanced location settings in the campaign settings, and correct that.