I am a provider new to TER and I saw that there is an Austin Ad Board and I do outcalls to Austin so that's fine for now but is there a San Antonio Ad Board that I can post on? If so, I can't find it...

Texas gets 3 cities, which isn't too shabby.   Many states get none.
 

If it were me I’d post in Austin and mention in the ad title that you’re located out of San Antonio. I’m reasonably sure that the surrounding cities would be using the Austin board.  

So are San Antonio/Austin. I know it seems like SA should be the one getting "top billing" as it's the larger city of the two, but it is what it is.