In a slow cooker, mix together jalapeno, garlic, chili powder, oil, and lime juice. Add chicken and stir well to coat. Cook on low for 4 to 6 hours. Remove chicken and shred, discarding bones (if any). Return to slow cooker until ready to serve. Use a slotted spoon to serve.
Top tostada shells with chicken, lettuce, cheese, salsa, and sour cream as desired.