Cars.com allows people to search for trucks within a specified radius of a ZIP code, which allows for searching of a large portion of Texas. Craigslist also has subpages for most areas of Texas, which allows people to search in the state's metropolitan areas.
Both Cars.com and Craigslist allow people to search for trucks sold by both dealers and private sellers. Another option is to look at local truck dealerships, which are fairly common in Texas due to the popularity of trucks in the state. Some dealerships, such as North Texas Truck Stop, list trucks for sale and allow potential buyers to shop online through eBay or other websites.