Combine the cilantro, lime juice, oil, chile powder and garlic in a large dish. Add the pork slices and turn to coat; let sit for 15 minutes at room temperature. Preheat a grill or grill pan over medium-high heat.
Remove the pork from the marinade and discard the remaining marinade. Sprinkle the pork all over with 1/2 teaspoon salt. Grill, turning once, until browned and still a little rosy inside, about 10 minutes. Transfer to a large plate.
Grill the tortillas, turning once, until grill marks appear and the tortillas are softened, about 1 minute. Transfer to individual plates. Spread each tortilla with some sour cream. Divide the pork and avocado among the tortillas. Top each taco with salsa and sprinkle with cilantro sprigs. Serve with lime wedges.
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