"Sumac can be found in the spice section in most Middle Eastern markets. It is usually sold in the dried berry form, or ground. And while the sumac spice is not as well known in the West as it is in the Middle East, more and more Western cooks are being introduced to the lemony spice. Like their Middle Eastern counterparts, it is no surprise that they too have marveled over its endless uses as a seasoning. There really is no spice like it."
Here's a good link that tells all about the spice....
Im looking for a sub. for it but Im not sure there is a good one.