Brown turkey and onion in large skillet over medium heat, stirring to separate meat. Combine turkey mixture, tomato paste, salsa, cilantro and salt in slow cooker. Cover; cook on low 4-5 hours.
Just before serving, melt butter in small saucepan over low heat. Stir in flour and salt; cook 1 minute. Carefully stir in milk. Cook and stir over low heat until thickened. Remove from heat. Combine sour cream and sprinkle of ground red pepper in small bowl. Stir into hot milk mixture. Return to heat; cook over low heat 1 minute, stirring constantly.
To serve, spoon 1/4 cup turkey mixture into each taco shell. Spoon sour cream mixture over taco filling.