Spray slow cooker with non-stick cooking spray and place chicken breasts inside. Top with salsa and fajita seasoning and cook on low for 5-6 hours or 2-3 hours on high.
Remove the chicken breasts and shred the chicken with two forks. Return the shredded chicken to the crockpot, along with the sliced peppers and onion. Cook on high for 30 minutes or until the peppers are tender.
Spread refried beans on the tostadas, layer with chicken and pepper mixture, then top with your favorite fajita toppings (sour cream, tomatoes, cheese, salsa, avocado, etc).