Hi colorfinger,
Mattress covers aren’t tested for flame retardancy by themselves because the fire regulations require that the complete mattress is tested … not just a cover or any individual component. Different types of mattresses with different designs or materials may need different types of covers to pass the regulations so a mattress that passes with one type of cover may fail with a different cover.
Having said that … some of the better sources for individual covers that I’m aware of (with and without wool) … most of which have some type of fire retardant barrier (most of them are used with mattresses that have passed the regulations)… are included in the component post here.
Phoenix