Texas is the state that gets the most tornadoes on average in the USA. This is mainly because it is the largest state in Tornado Alley but Texas IS a state that get a lot of tornadoes and especially in the Texas Panhandle area and south of the Panhandle. Storms and tornadoes in Texas are the most common in early Spring (April and early May) so if you want to chase in Texas you should plan to chase around this time.
If you chase in Texas you will most likely chase, as mentioned, in the Panhandle area and south of the Panhandle. Not only is the Texas panhandle a hotspot for tornadoes is also offers a great flat chase country. Storms and tornadoes do happen in other parts as well of course, especially in the area north of Dallas, but some parts of the State are often left out by chasers because of the chase terrain being to difficult to chase in.
The two cities all tours are based out of are Dallas/Fort Worth and sometimes Arlington and Amarillo (in the Texas Panhandle). If any of the cities in this list is empty it is because there are no tours from this city this season.
Storm chasing is, by its nature, not geographically set: you go where the storms are. Even if you start a chase in Texas you can end up in neighboring states while chasing. If you specifically want to chase a setup in Texas or if you would like to chase the storms that exist in Texas on a specific day your best option is to try a Private Tour.
You will have to ask around among the tours in the list linked above to see who can help you out with a storm chasing tour in Texas. A good idea is to ask any of the tour companies listed above as this typically means they can easily do a tour from that city.