If it's just a small amount, like half a teaspoon or less, you could conceivably get away with substituting chili powder, which is mainly paprika along with other seasonings such as garlic, salt, cumin, and a bit of cayenne. It's slightly hotter than plain paprika, but not overwhelmingly so. Some other ground red peppers like ancho chili powder, chipotle powder, or hot sauce would also work. Chili powder will also suffice if the paprika is just being used for a garnish. With these spices, you can go with a 1:1 ratio of the substitute spice to the paprika amount needed.

Not all paprika tastes the same, but that doesn't mean that you can't use one that's different than what's called for in your recipe. For example, you can used spicy paprika in place of sweet, but the final product will have much more of a kick. Smoked paprika will impart a different flavor, too.