Sansho powder is a Japanese seasoning powder made from the ground berries of the prickly ash tree. The powder has a peppery-lemon flavor with a long residual heat. Sansho powder is used to season sushi as well as various Japanese noodle dishes.
You can purchase sansho powder in most any Asian market. One of the most prominent brands is S&B. You can buy this online at Amazon.com: Sansho Pepper Powder.
If you don't have sansho powder then you can use one of these substitutes: