Actually...Driving around is your best bet.
The rent houses, and I mean the really good ones not some fratrat dump, have really good landlords. They've been renting their houses in the lil' ol' town of San Marcos for so long that they don't have to advertise. The house usually gets passed down to friends, because if you have a great house with a good landlord people close to you will know about it. Then when the lease is up or you want to move there's always someone ready to jump on it. I know 4 or 5 landlords and none of them ever advertise except for an ocasional sign in the front yard.
I was working out off of hunter road between posey and centerpoint, there's quit a few neighborhood roads that go west off of hunter. Each one had at least one for rent or lease on it.
But my suggestion is to drive all over town, find an area you like and then start looking for signs.
The guy that I bought my house through, owns a few in town, his name is Clarence Miller @ Hart Properties, he's pretty cool I guess. The other guys I know, never have vacancies.
hope this was some help.