Heat oven to 225° F. Wrap the tostada shells in foil. Place in oven until warm, about 6 minutes. Meanwhile, warm the beans in a small pan over medium-low heat. Place the chicken in a nonstick skillet and warm over medium heat, turning once, for 4 minutes. Place 2 tostada shells on each plate. Divide the beans, chicken, lettuce, sour cream (if using), salsa, and cheese among the tostadas.