In a small bowl, combine sour cream and chipotle sauce until thoroughly blended. Refrigerate until ready to use.
Sauté the red pepper over medium heat in oil until red pepper is tender; add cooked, taco-seasoned ground beef.
Stir in chili sauce, salt, and pepper.
When heated through, remove from heat and set aside.
On each tostada shell, layer lettuce, taco meat mixture, guacamole, cheese, and chipotle sour cream.
Choosing avocados: For immediate use, avocados should be firm but have some give when you squeeze them gently in the palm of your hand. I usually buy the Hass variety, which turns darker as they ripen (other varieties may not darken with ripeness). The color of this variety in addition to the feel when you give them a gentle squeeze will help you determine the level of ripeness. If you end up with unripe fruit that you need to use, you can hasten ripening by putting them in a paper bag and rolling down the top. Leave them in the bag at room temperature until they ripen, checking them each day with a gentle squeeze.