Preheat the oven to 375°F (190°C).
In a medium bowl, combine shredded chicken, olive oil, ground cumin, chili powder, garlic powder, onion powder, salt, and black pepper. Mix well to evenly coat the chicken with the spices.
Place the corn tostada shells on a baking sheet and warm them in the oven for 5 minutes until slightly crispy.
Spread 1/4 cup of refried beans evenly over each warmed tostada shell.
Divide the seasoned shredded chicken evenly and spread it on top of the refried beans on each tostada.
Return the baking sheet to the oven and bake for 10 minutes until the chicken is heated through and the tostadas are crispy.
Remove the tostadas from the oven and top each one with shredded lettuce, diced tomatoes, and shredded cheddar cheese.
Drizzle about 1 tablespoon of sour cream over each tostada.
Serve immediately and enjoy your chicken tostadas.