Blake,
If you really want to find an indigo, you will need to go south of San Antonio, but not very far. They occur in the scrub country just south of town. They are easiest to see driving quiet country dirt roads that run near watercourses. They are active first thing in the morning until it gets too warm. In October, they might be active later in the day as it won't be as hot.
October nights are often just at the border of too cool for roadhunting, but if the mornings are cool enough, flipping stuff can be productive. Look around empty fields, etc, within the city for trashpiles. South and east of town is better than north and west for flipping.
Also in October, you can supposedly find TX Alligator lizards crossing quiet country roads in the steep hilly canyons west of town. They are generally active just before dusk.
With that all said, you should be aware that TX has a new law that prohibits hunting on a road or right of way. This technically outlaws roadhunting for these species. Whether this law is going to be enforced is another issue. If you are only taking photos (no bags/hooks) you can do it legally. If you are taking animals, you are hunting.
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Chris Harrison
San Antonio, Texas