It’s a simple sauce made with soy sauce, mirin, sugar, and sake (no eel is used in the sauce except for the more traditional nitsume sauce, which contains eel broth), that’s thick enough to be used as a glaze.
You can purchase eel sauce in Japanese supermarkets such as Mitsuwa, Sunrise Mart, and Nijiya Market. You may also find eel sauce in some Asian supermarkets although it’s not a guarantee that you will find it.