My feeling is that presumably if you were able to purchase a kidney or other vital organ, you should not hesitate to do so (assuming that it is a cclear case of pikuach nefesh). Pikuach nefesh is doicha Shabbos, it certanly should be doicha dina dimalchusa.
That is only my feeling and obviously a morah horah would have to be asked. If it would lead to people having their organs forcibly removed, I can see that being a reason to prohibit it l’halach.
However, I seem to recall that there was a statement by Rav Elyashiv Shlit”a about a year ago regarding a case dealing with organs from China, I do not recall if they were forcibly removed, and thus my speculation would moot because there was a p’sak regarding the issue.