Check out 58.com or ganji.com, they also have mobile-apps. There you can find all kind of 2nd hand stuff for every chinese city. Bought my last e-scooter there, good price and just 2 months old. If you buy 2nd hand, insist on original invoice and also make a picture from the sellers ID-card, or else you could end up with a stolen bike and problems following. Good hunting!
The OP is right, i also wanted to refresh tetanus shots and where told by several doctors that it is not recommended in China. They will give you a shot, if you happen to get injured but not in advance. I wonder what if there is no doctor near and one does have an accident.
Will try to get the shot's done in the next two weeks, let's see if it's possible.