Does anyone know where you can purchase GAP insurance for your car if your auto insurance doesn't offer it?

I bought a 2006 auto a couple weeks ago and when I called my insurance co. to add it, they said they could add GAP insurance, too. (which I need) Now they are saying they can only add it to a NEW car, not used. Anyone know if I can purchase strictly a GAP policy to cover my car? I don't want to switch insurance because of discounts having my other car, homeowners, etc. through them.