Chaat masala is the popular spice blend used primarily in the style of Indian street food called chaat. Aside from that, it is sometimes sprinkled on fruit or on vegetables. This blend is considered to be milder than other Indian spice blends like tandoori masala despite the fact that it contains chili peppers. You can usually find multiple brands of prepackaged chaat masala at Indian food stores. If you do not have these stores near you and need it quickly, there are options when it comes to chaat masala substitutes.
Your best bet: Make your own chaat masala
Garam masala is another Indian spice blend that contains many of the same flavors as those found in chaat masala. While blends can differ depending on regional preference, both typically include cumin along with coriander and ground chili peppers. Garam masala translates to hot spices; while it is not always hot, it usually is. That heat is part of why it makes a good chaat masala substitute. It does lack the acidity that mango powder provides to chaat masala, but you can use citrus juice or another mango powder substitute in its place. Note also that cumin and coriander are sometimes toned down in chaat masala, whereas their flavor notes will be more discernible in garam masala.
Use garam masala as a 1:1 substitute for chaat masala.
In a pinch: Harissa powder or paste
Harissa is a North African spice blend that shares some of the same ingredients with chaat masala, which means that it also shares some of the same flavor profile. Like chaat masala, most harissa powder blends are built on a foundation of chili peppers along with cumin and coriander. Harissa paste will typically have the same set of ingredients, but may also have tomatoes and mint.
Note that harissa powder blends are usually hotter than chaat masala, so bear this in mind and adjust your measurements accordingly. Start with about half the amount that your recipe requires for chaat masala and increase to taste from there.
Curry powder blends typically contain both cumin and coriander and some will also have chili peppers among their spices. The most obvious difference between curry powder and chaat masala is the use of turmeric. Turmeric gives curry powder its bright yellow color and adds an earthy note to its flavor. While this should not be a problem in many recipes that require chaat masala, it may not be a welcome addition in some.