Spray the insert of a 6 qt or larger sized slow cooker with cooking spray.
Place the chicken breasts into the crock of the slow cooker.
Sprinkle on your taco seasoning and pour the entire jar of salsa over the top.
Put on the lid and cook on high for 3-4 hours, depending on the size of the chicken breasts and the cooker you're using.
Check for doneness around hour 3. The chicken is done when it registers 165° on a meat thermometer. If you don't have a thermometer, cook until the chicken is no longer pink in the middle, the juices run clear, and the chicken can be easily shredded.
Once cooking has finished, using two forks, shred the chicken. Turn off the slow cooker or leave it on the "Keep Warm" setting and allow the shredded chicken to sit in the juices for at least 15 minutes.
Add in the sour cream and stir until well blended.
Serve with tortillas for slow cooker tacos, or as a topping for nachos, or a filling for burritos.