I don't know where you are in Texas, but you might be able to gather some in the wild. Here's it's distribution in the state:

You also might check with the Texas division of the Federal Highway Administration. It's on their list of authorized roadside use plants.

Finally, check the suppliers page at the Lady Bird Johnson Wildflower Center, linked below. It's on their list as well, and they may be able to lead you to a source for seeds.

Here is a link that might be useful: Lady Bird Johnson Wildflower Center