China has finally admitted that most of its transplantable organs come from executed prisoners, and many of those are sold to foreigners. The LA Times quotes one kidney-seeking American who went to Guangdong for a quick replacement, and it's fairly clear that other Americans have benefited from the harvest.
Gruesome tales of medical tourism tend to turn people against organ markets, but it's worth noting that this is the kind of insane desperation the current donation-based system creates. Given other options, China's prison population is pretty much the last place you'd want your kidney to come from. Stateside, American prisoners who willingly donate are considered undesirable by donor recruitment agencies. Part of this reluctance is class prejudice and racism, but part of it is the often-sketchy social histories prisoners are assumed to have. The truth of transplantation is that you can't test for everything; it's possible, for instance, that a donor has been exposed to HIV so recently that the antigen can't be detected. So donors with histories of risky behavior are weeded out based on self-reports and information provided by families. Intravenous drug use and certain sexual behaviours count as a risky; I'm pretty sure doing time in a rural Chinese prison would too.
In general, kidney production is not something people want to outsource to countries where they won't touch the drinking water. But if the options are restricted to buying a random organ or dropping dead, people are going to choose the former.