If you can't get Za'atar, it is easy to make it yourself. There are are many different varieties, here is one with sumac:
3 tbsp sumac
2 tbsp oregano
1 tbsp savory
1 tbsp majoram
1/2 tsp salt
1 tbsps sesame seeds
Put the spices in a food processor and give them a short whizz. Dry roast the sesame seeds and roughly grind with a pestil and mortar. Keeps for a couple of months.
Great on flat bread with a bit of olive oil.