We go to mexico all the time and I never exchange my dollars. Most people actually prefer the dollars to peso. Plus it cost more to exchange the peso back for dollars.
I feel more "cultured" if I exchange my money, besides it feels like I have so much more money after the exchange, lol.
I searched google "currency exchange in san antonio" and appear to have found where you need to go. It's a place called Bonham Exchange. LOL what the heck right?