Acai

What is an acai berry?  Why can't you buy them in a store, but you find acai juice in stuff like vitamin water?  I don't recall ever hearing of acai until about 3 years ago - introduced as some 'super fruit'.  I heard that and thought of one of those fruit-of-the-loom guys as a super hero.  It's Mighty Mango - to the rescue!!  I've never had a raw acai - are they even edible, or is it just the juice that you want?  Perhaps one of these days I'll find an acai bush (tree?) and try one.