Sandy they are called Purple Fall Aster or Aromatic Aster,
Symphyotrichum oblongifolium, are these the ones you are talking about?
They should have them in many nurseries. http://davesgarden.com/guides/pf/go/76317
It's the purple ones/ drought and heat tolerant that I am looking for. Thank's so much, Josephine. However, the references to the white ones have given me an idea to combine the two. Does anyone have experience with Stokes Aster? They are said to be good by the catalogs, but that is often not applicable to South Texas.
Wildseed Farms in Fredricksburg, TX sells seeds for a lavender-blue daisy that they call Tahoka Daisy / Prairie Aster, Machaeranthera tanacetifolia (Asteraceae). Sorry, I do not know how to do links to their site. Just enter www.wildseedfarm.com and go to the list of Individual Species of Wildflower Seeds.