Hi bbunney13,
I would keep in mind that most mattresses don’t use chemical fire barriers and some even use natural materials such as wool to pass the fire regulations. There is more about “safe” fire barriers in this article and post #2 here and the posts it links to at the end.
If you wish to purchase a mattress that has no fire barrier at all (which means it hasn’t passed the fire regulations) … then any licensed health professional can write a prescription that will allow you to purchase one. In most states it can also save you the state taxes (see post #6 here). Many manufacturers can also make their “regular” mattresses without the fire barrier.
Most manufacturers that are willing to make a non compliant mattress without a fire barrier will provide you with a form that includes the wording that you just need to have signed by your doctor or licensed health professional but in most cases it’s as simple as a prescription that says something like “this patient requires a mattress without fire retardants”.
Phoenix