Unless the law has been changed in the last year, the above is incorrect, at least on a federal level. Some states may have more restrictive requirements, but Texas is not one of them.
Flame retardant is required only for pajamas and flannel intended for childrens use.
It is also available for some pajamas for adult use. You can buy untreated flannel at any fabric store. Not all of it is untreated, and you have to ask for it, but you can easily get it. Last time I bought any was a year or so ago. If the laws have changed I'm unaware of it.