4 things you could look into......
Are you totally sure they don't sell it anywhere in China? We have very few people who take the USMLE but I still found it sitting there in my university book store. I mean, it's a cool book. Even if you were NOT taking step 1, it would be a nice review of what you learned in the basic science years.
Buy it from an older student. Or ask someone who took the USMLE how he got his (if you want the newest version, which is kind of important).
Can you get a money order made (or international cheque or something like that)?
Call blackwell scinece publishing or write them an email. If they want to sell books (and they DO), they'll be able to give you the best advice.
Good luck.
ps. You need more than First Aid for the tests. The hard part of being a foreigner taking the USMLE is not knowing what to study, it's knowing what NOT to study.