One company that offers this insurance is Purina. Purina's insurance division
Pet Insurance: Pet Health Insurance Quotes from PurinaCare is headquartered right here in San Antonio.
Purina pitches pet insurance for dogs, cats - Daily Journal Online Archives (http://www.dailyjournalonline.com/articles/2008/06/11/news/doc484fe0ee55b6c820946015.txt - broken link)
I don't use it or know anything about it other than that. Since this isn't really a San Antonio topic, I'm going to move this thread to the Dogs forum where it is more likely to be seen by folks who can answer your questions.