Black sesame seeds are sesame seeds in black color. It is believed their benefits to the body is different than the white sesame seeds, which looks brown to me.
In Asian culture, there are sweets that made with black sesame seeds, very tasty if you like sesame and sweet. Not the same purpose as the drink that we are talking about here. Just some information regarding sesame seeds for you.
Health food store might not carry them. Most Asian grocery stores do carry them since it is use in food preparation.
As suggested, try to locate it on-line if there are no Asian grocery stores close by.