It is important to differentiate between zante currants, which are made from grapes, and black currants, which are a totally different thing. I bring this up because I was confused about this for quite a while, and others might be too.
It's black currants that I think you are referring to. I haven't used them and yes, they are VERY difficult to find in the U.S.. Here is the only place that I know to get them: www.currantc.com