Books are what started me with my mask fetish, I'm pretty sure (though I'm not certain, and I would love to be able to time travel to whatever point my fetish started, and then whatever point it transitioned partially to protective medical masks from anesthesia masks).
But later on, I did start watching medical documentaries, and often there would be a teenage girl on there (similar in age to me) who, because of a transplant, would need to wear a mask on the floor and in public. At times, she would be in her hospital gown, which never turned me on, but at other times, it was just comfy clothes and a mask.
I have also watched an air disaster movie where the yellow passenger oxygen masks fell and a pretty teenage girl put one on and did the straps, and that was a turn on.
But yeah, books started it all, and medical documentaries nurtured it, and now sites like this and Pink Toxins, which have pics but more importantly areas to talk about our fetishes. I love the psychology of sexuality.