Preheat your oven to 400°F. Arrange the tostada shells on a baking sheet and bake for about 5 minutes, or until they are crispy and lightly golden.
In a medium bowl, combine the shredded chicken and barbecue sauce. Mix well until the chicken is evenly coated.
Remove the tostada shells from the oven and let them cool slightly.
Top each tostada shell with a generous portion of the BBQ chicken mixture.
Add shredded lettuce, diced tomatoes, and shredded cheddar cheese on top of the chicken.
Drizzle sour cream over each tostada and garnish with sliced green onions and avocado.
If desired, sprinkle chili powder, salt, and pepper to taste.
Serve immediately and enjoy your delicious BBQ chicken tostadas!