I don't remember the brand, but it's in a little bottle near the Vapo-Rub and that stuff. It's an oil, so it's like 5 or 6 ounces. It doesn't go in the water because it would gum up the motor; there should be a little reservoir kind of thing near where the vapor comes out to put it in. I think it was under $5 (like $3.99) the last time and we had it all season. You could also check out Whole Foods, it might cost a little more but it would probably be a really good quality essential oil. Be sure your machine can handle it.