For souvlaki, mix first 5 ingredients to make a marinade and set aside. Distribute lamb, bell peppers, onion and tomatoes among 8 skewers. Place in a large shallow pan with marinade. Cover and refrigerate at least 3 hours, rotating skewers once. Heat a gas grill to medium, or use a charcoal grill and allow the coals to turn a light-ash color. Coat grill with cooking spray. Grill until lamb is medium rare and lightly charred, 5 to 8 minutes. Rotate to cook evenly. To cook in oven, broil on metal skewers 6 minutes, rotating until evenly browned. Grill or toast pitas. For sauce, puree cucumbers and a pinch of salt in a food processor. Place in a fine-screen colander and squeeze out the juice. Reserve juice and discard pulp. Whisk cucumber and lemon juices in a bowl with yogurt. Mix in garlic; season with salt and pepper. Serve with pitas and skewers.