In a bowl whisk together olive oil, garlic, cumin, salt and cayenne pepper (if using). Add in shrimp and toss to coat completely. Cover and refrigerate for 20 minutes to give the flavors a chance to marry.
Cook shrimp in a skillet on medium heat until pink and cooked through, about 5 minutes. Turn off heat and cover to keep warm.
Use enough oil to lightly coat the bottom of a small pan, about 2 tablespoons. Heat over medium-high heat. Cook tortillas until soft, about 30 seconds on each side. Fold over to make a taco shell. As you cook each tortilla, place them on paper towels to absorb any of the oil left over.
Stir all ingredients for the Cilantro-Lime Sour Cream together in a bowl.
Spoon 5 shrimp into each taco shell. Top with lettuce, tomato, avocado and cilantro-lime sour cream. Serve.