Wasabi paste is a ready to use mixture of dried horseradish, mustard, food coloring and water. This is typically not a "pure" real wasabi product but it is commonly used as a condiment with sushi.
Look for wasabi paste in the Ethnic food section of your grocery store or any Asian grocery store. You can also purchase it online at Amazon.com: Wasabi Paste
If you don't have wasabi paste you can substitute (per tablespoon needed):
1 tablespoon Wasabi powder mixed with 2-3 teaspoons warm water OR> 1 tablespoon hot mustard powder (such as Coleman's) mixed with 1-2 teaspoons water OR> Use tablespoon prepared horseradish OR> 1 tablespoon freshly grated horseradish