Interesting. I've had those in "Chinese" Dim Sum restaurants, but while they were clearly atypical for Chinese food, I had no idea they were originally Portuguese.
Strongly second the recommendation. They're delicious!
Custard tarts derived from the Portuguese pastry were
introduced in Hong Kong in the 1940s by cha chaan tengs
via the Portuguese colony of Macau.
Other places where for sure you can find them:
Paris (and other cities in france), Brussels, Luxembourg, Newark (US), Toronto, Petersham (Sydney), Poland (at Biedronka, a Portuguese supermarkets chain), Macau and other Portuguese ex-colonies.