In a small bowl, combine the first eight ingredients, and mix well. Place the pork shoulder in a slow cooker and smear half the spice mixture over the top side of the meat. Flip it over, and rub the remaining spice blend onto the other side.
Combine the salsa and water in a small bowl or glass measuring cup, and pour it around the pork. Cook on low heat for 8–10 hours, or high heat for 6–7 hours, or until the meat easily pulls away from the bone.
Transfer the pork to a plate and remove the bone and any excess fat. Shred the pork into small pieces using two forks, and place it in a serving bowl. Add the liquid from the slow cooker and stir to combine.
To make this dish vegetarian, omit the pulled pork and serve the tostadas with the pinto beans as the main protein. For the beans, heat two cups with 1 teaspoon chill powder, 1 teaspoon ground cumin and 2 tablespoons of oil until warmed through. Season with salt and pepper, if desired.
The pork can be made one day in advance. Let it cool completely, cover it with aluminum foil and store it in the fridge overnight. To reheat, place the pork and sauce in a saucepan and gently warm over low heat.