There really isn't a substitute for sumac. You can find it at a store that specializes in middle-eastern foods. Our local coop carries it. Here is a great article about sumac explaining it's unique properties.
http://www.seriouseats...

I adore sumac, so I get it in large quantities at ethnic markets. Our local farmer's market has a spice stand where you can find it in smaller quantities. If, however, you just can't come up with any, you could reasonably use finely chopped lemon zest. It's not the same, but it does add the zesty perk that sumac would. (Jamie Oliver told me it was ok to sub lemon zest for sumac. And I trust Jamie.)

You can also order it online in small quantities. I've had very good customer service from https://www.worldspice....