They count among their clients government officials and employees of state-owned enterprises, for whom a second child would be a fireable offence.
Members of the Chinese Communist Party would also face disciplinary action if a second child were reported.
But others want tall, Eurasian children, agents said.
"Lots of clients that are Chinese do use tall blond donors," said Jennifer Garcia, case coordinator at Extraordinary Conceptions, a Carlsbad, California-based agency where 40 per cent of clients are Chinese.
Obtaining a Chinese household registration, which is necessary to enjoy subsidised health care and enrol for lower tuition as a local student in state schools, is more complicated, if not impossible for second children.
For children who are foreign citizens, parents must apply for visas and residence permits.
"You can basically make a designer baby nowadays," said Garcia.
Friends began to ask him to help them do the same thing and in 2012, he set up his own agency, Di Yi Consulting.
Seeking surrogacy overseas is not in itself illegal, and Chinese surrogacy agency websites, often adorned with pictures of chubby infants, highlight the possibility of bespoke babies.
Chinese surrogacy clients typically want to use their own eggs and sperm, which allows them to have a child who is fully biologically theirs, agents said.
They turned twice to domestic surrogates after struggling and failing to conceive on their own.
Both attempts were unsuccessful, and left them unimpressed with the impersonal nature of in-vitro fertilisation (IVF) treatment in China.