In a medium mixing bowl, combine the buttermilk, mayonnaise, sour cream, lemon juice, garlic powder and onion powder. Whisk the mixture together until smooth. Add the blue cheese and mix just until blended. Season with salt and pepper. Chill in an airtight container in the refrigerator for at least 2 hours and up 2 days before serving.
Cut the bacon into 1/4- to 1/2-inch pieces and cook them in a large skillet over medium heat until brown and crisp, 5 to 7 minutes.
Core and quarter each head of lettuce and divide among 12 salad plates.
Serve a generous dollop of dressing over each lettuce wedge. Top with the crumbled bacon and diced tomato.