Tortillas are uncommon in Japan. They can be found at import stores or Costco, or bought online, but they can be quite expensive. (2015 update: I am seeing tortillas in large supermarkets more often now, as Mexican food is slowly becoming more well known.) My solution was to learn to make my own. I started with flour tortillas, and found it easier than expected.
Ingredients (makes 8 tortillas)
1-1/4 cups (160g) all-purpose flour
1-1/4 tsp baking powder
1/2 tsp salt
1/4 cup (50g) shortening
1/4 to 1/2 cup (60-120mL) hot water
Mix the dry ingredients in a large bowl. Add the shortening and cut in with a pastry cutter or two knives. Add the hot water little by little and stir in.
Knead dough 30-40 times. Let rest 1 hour.
Divide into 8 equal pieces. Let the pieces rest 20 minutes.
Roll out with a rolling pin (or press on a tortilla press) to make 8-inch (20cm) diameter circles. Cook in a dry frying pan over medium heat. It should take just a minute or so on each side to cook through and become brown in spots.
While cooking the remaining tortillas, cover the cooked ones with a towel to keep from drying out.