The sauce, easily available in Europe, but not in US or Canada, is Peking Duck Sauce. The one generally cited is Lee Kum Kee Peking Duck Sauce. Do not confuse with Saucy Susan Peking Duck Sauce or other generic Duck Sauces, that are orange or yellow, which are a kind of pineapple sweet and sour sauce. A closer match is Hoisin Sauce, similar but not quite the same if one is very particular; three varieties are available in US (all at Amazon.com); the ones that I have found are Lee Kum Kee/Panda, Koon Chun (very good), and Wok Mei. I have found a source at Amazon.co.uk, but with shipping to the US, it costs about 23 USD. I have to rely on the web, as there are no Chinese groceries within a hundred or more miles of my residence; traveling to which would cost more than the shipping from England. I hope this helps.