Sure, you can buy a bottle of teriyaki sauce, but this is pretty quick and easy to whip up. This makes about 1 cup, and you can store it in the fridge for about a week or so. 

1/2 cup soy sauce or tamari
1/4 cup brown sugar
1 1/2 tsps. fresh ginger, minced
1 tsp. garlic, minced
1 tbsp. honey
1 tsp. sesame oil
3 tbps. mirin
3 tsps. corn starch
1/4 cup water

Mix the water and corn starch. Heat all the ingredients in a saucepan. Bring to a boil, then reduce the heat and simmer for about 4 minutes and let cool.