I use dried jarred dry harissa spice from Whole Foods (in the spice section). If you don't have any, sub in a mix of chile powder, coriander, garlic, cumin, cayenne, and salt. (Or really, any spices you like - a curry blend would be delicious here too). You can also use jarred harissa paste instead of dry, just be sure to add it to taste since spice levels can vary.
Vegan option: skip the yogurt and just make a paste from the olive oil and harissa. Add it into your pita to taste (it can be spicy).