Thai mang da, known in Vietnamese as ca cuong, is available in the U.S. in the form of an artificial extract. It has a pleasant floral aroma with a slightly funky background scent. It is an essential flavor in North Vietnamese cuisine. Former residents of Hanoi put a drop of the stuff in the dipping sauce with a little glass eyedropper when they eat banh cuon.
Authentic essence of mang da is made from an increasingly rare water bug (Lethocerus indicus) that looks almost exactly like the large brown insect we call an American cockroach. Mang da extract is made from the insects' musk glands. In Thailand, it is used to make a pungent chili sauce called nam prik mang da.
The first mang da sauce I ever encountered was the extremely aromatic Laotian variety. It was served with steamed fish at a Laotian restaurant in the north of Thailand. The thick black gunk smelled stronger than blue cheese. I wasn't very fond of it at the time. But the artificial stuff in the glass bottle is downright pleasant. So now I find myself looking for a bottle of mang da essence to keep in my car in case I run across some North Vietnamese food.
I am guessing they sell it at Hong Kong supermarket.