I have terminal brain cancer. I am on SSI but I own child support. How can I get free legal help? I live in San Antonio Texas.

You don't get any special legal help, you can contact your local legal aid society and see if they offer that help
But what type of help are you looking for ?

You say you 'own' support and we can't tell if you are owed it or owe it.
If you owe it and are dying, you die owing, that's all. Let the other parent know.
If you are owed, then ask your state worker who handles your SSI.
San Antonio is big enough to have advocates who can help you go back to court to take the next step to collect, and also to work out arrangements for the child or children.