Place the taco shells on a baking sheet and warm in a 200 degrees F oven. Meanwhile, heat the oil in a large non-stick skillet over medium-high. Add the onion and bell peppers and cook until crisp-tender (about 3 to 4 minutes). Divide and spoon the onion/pepper mixture into the taco shells. Top each with an equal amount of lettuce, cheese, salsa and shrimp. Serve immediately.