The best substitute for Aleppo pepper, or Halaby peppers, is a mixture of sweet paprika and cayenne pepper. Aleppo pepper has a mild heat, so you can use about 4 times as much paprika as cayenne pepper.
In a hot skillet over medium heat, toast the paprika & cayenne pepper until they develop a smoky aroma. Turn frequently. Be careful not to burn.
In the bowl of a food processor add the chillies with salt. Process until a powder forms.
Scoop out using a spatula & store in an airtight container or a mason jar in a cool dark place. You may also keep this in the fridge.
Notes
Think of this recipe as a guideline. Very the quantity of paprika and cayenne from your cupboard for the zing suitable for you. In a mason jar, they make great gifts!