Place the chicken into a large pot along with 1/4 cup of sliced onion, garlic and enough water to cover. Bring to a boil and cook until no longer pink, about 25 minutes. Remove chicken; cool and shred meat. Discard skin and bones. Heat oil in a large skillet over medium heat. Add the remaining onion slices, and fry until transparent. Place tomatoes into a pan of boiling water (you could use the leftover chicken water), and boil until they split open. Place the tomatoes, the whole can of chipotle peppers, and chicken bouillon into a blender or large food processor. Process until smooth. Season with salt and pepper. Pour this mixture into a large pan, and add the shredded chicken and cooked onions. Heat through. To serve, each person spreads some sour cream on their tostada shell, and tops with a spoonful of the chicken.