it's so simple to make yourself, there's no reason to buy the dressing pre-made, just get the sumac.
¼ cup lemon juice
1/3 cup olive oil
1 tablespoon white wine vinegar
1 teaspoon sumac powder
1 large garlic clove
salt & fresh pepper to taste
Smash the garlic clove with a pinch of salt and mash using a mortar and pestle or the back of your knife until you have a paste. Add to a bowl with the lemon juice and sumac, and whisk in the olive oil. Season to taste with pepper.
you can play with the proportions a bit to suit your taste, and even add some toasted ground cumin or chopped fresh mint if you feel like it.
if you *really* want the dressing you had at the restaurant, you can always ask if they'd be willing to sell you some!
When I make it, I use the same ingredients and way more garlic and less oil than this recipe I like mine more tart and garlicy. I also put parsley, not mint, in my fattoush salad, and I think that it tastes most authentic this way. You can get the sumac at Hiller's Market locally, that's the only place I've seen it outside of an Arabic grocery store. Or order online at Penzeys.