This is a site for second-hand computers of all kinds in Shanghai:

http://itbbs.pconline.com.cn/es/

Here are the five major areas in Shanghai where second-hand computers of all kinds are sold.  I have left the answer in Chinese as you will need the assistance of a Chinese-speaking person on how to get there and how to bargain once you are there:

A.  浦东二手电脑上海市浦东新区B.  QQ二手电脑总汇上海市闵行区C.  小蒋二手电脑专卖吴宝路958号D.  兴达二手电脑吴宝路新龙村观音堂38号E.  恒宇二手电脑总汇上海市

I think you can get one on Baoshan road, take metro line 3 o 4 and alight at baoshan road and ask anyone where is the baoshan market, you can also take line ten and interchange line four at hailun road