Your best places will generally be in ALL the areas south of the Carrizo springs to Beeville line. The furhter south you go, the more snakes you'll find. I usually hang out around places like Hebronville (and all nearby roads), La Gloria,Benavides,Freer and a lot of other towns about 3-10 miles inland from HWY 83 from the Laredo to Brownsville stretch.
I have found: Lots of Diamondbacks and bullsnakes (they get HUGE there, found 6 foot plus of both), Desert Kings, glossy's, longnoses, milksnakes, checkered garters, great plains ratsnakes, texas ratsnakes, ruthven's whipsnakes, coral snakes,and quite a few Indigos (protected), one cat-eyed snake(protected), and a few other smaller species. Remember road hunting is illegal and you'll need a hunting license to collect snakes. You can cruise down roads and look for favorable spots get out and walk a little. That works pretty well, if you have a good flashlight!
There are also some nice lizards you can find there (if interested) Blue spiny (found one huge blue male 14.5" long), great plains skink, texas banded geckos (at night on roads), mediterranean geckos (on almost every single building at night), texas spiny, reticulated collard (protected),whiptails, some stray spiny tailed and green Iguanas (mainly around Brownsville) and a few others.
Just avoid going there in the summer (even in your area it gets worse then), snakes tend to estivate (summer hibernation) for up to 2.5 months during hot and dry weather, usually around July to September are bad times to go. All the times I've went I've never been disappointed. I've always found something. Out of all the many different states I've been herping in (in my 30 plus years),this is by far the best spot in the US for me! I always enjoy going down there!
Good luck!
Zee