The taste of sumac is quite acidic, but there is also a currant-like background. A bit like currants mixed with lemon.
It is used in Middle Eastern cuisine - I think one of your fellow Americans will have to tell you what shop is easiest. There are some plants that are called sumac but whose berries are inedible - don't pick your own berries, buy them from someone who can confirm this is the kind you use in cooking.