Harissa substitutes aren’t difficult to find, and you may even have some in your spice rack right now! They closely mimic the flavor and impact in a dish that harissa boasts.
Harissa is a red chili paste blend that is popular in North African and Middle Eastern cooking. This blend includes Baklouti pepper, red pepper, and serrano peppers, as well as other spices like garlic, saffron, and coriander. It is typically smoky in terms of flavor and can be bought premade, usually mixed with olive oil as a preservative. It is considered the national condiment of Tunisia, where it is used as a flavoring for meat dishes, stews, and couscous. It is also closely associated with Moroccan flavors, particularly as a condiment for tagines. 
While it is a very popular condiment in certain parts of the world, it can still be considered exotic in other locales, and therefore hard to find. If not available at your grocer, or at a local Halal store, there are several spices that can be used to substitute for harissa in recipes including chili powder blend, berbere spice blend, sriracha sauce, and red pepper flakes.
Chili Powder Blend
This common blend of peppers is the easiest thing to grab from your cupboard. The flavor profiles of the blends are similar, since both incorporate mostly red chilis, along with some garlic and onion. Chili powder usually contains other spices, like cumin, but the overall taste will be the same. However harissa is much hotter and spicier than chili powder, so you may want to boost the heat with some extra red pepper flakes or cayenne pepper. 
Berbere Spice Blend
Berbere spice is a blend of peppers that are commonly used in Ethiopian cooking. In addition to chili peppers, it also usually contains ginger, garlic, nigella, basil, and fenugreek. Because of its closeness in origin, the taste of berbere spice will usually work well in recipes calling for harissa. Unfortunately, it can be as hard to find as harissa paste.
Sriracha has gained popularity all over the world as a common hot sauce. Originating in Thailand, this sauce is a paste of chili peppers, vinegar, garlic, sugar, and salt. It is thinner than harissa and has a much more intense, overwhelming heat. Adding some cumin or coriander will improve the depth of this sriracha flavor. 
Red Pepper Flakes
Red pepper flakes do not add the same depth of flavor and complexity as harissa, but they will add heat to your dish. If possible, add the red pepper flakes to either chili powder or sriracha to get the full intensity. Also, bear in mind that using flakes instead of a paste will change the consistency of your dish.