Tonkatsu sauce is like Worcester-base sauce used mainly for Tonkatsu (deep fried pork). Japanese Worcester sauce is different from American (English) one, and it has more fruits and vegetable such as tomatoes and apples along with sugar, vinegar,and salt. Tonkatsu sauce has similar favor to Japanese Worcester sauce, but it is much thicker.
Tonkatsu sauce can be a substitute for Okonomiyaki sauce.
It is available at Japanese markets or online stores.