A friend bough a pair of aluminum ones at a local large pharmacy store.  He went to one place, and was referred there to this other place.  Suggest you try at large pharmacies first, and if they do not have them, ask where you can buy them.

Most large pharmacies have them in Shenyang. There is also a large area in Shenyang where all the pharmacy wholesalers are to be found. They sell to the public and in that area surgical aid stores as well as medicines and TCM products can be found. I would think that most large cities have areas like this and the products are cheaper there. Shenyang is the only city that I am very familiar with and there seems to be different areas of the city where specific wholesale market areas for each type of product can be found. (eg hardware, shoes, cloths, spices, confectionery, furniture, etc) If you locate the area for the product you want you will not only get the best price but will have a wider selection available all in the one area.    

In Beijing, you can find many stores outside Beijing Jishuitan Hospital, http://www.jst-hosp.com.cn/Default.aspx?tabid=738, a hospital specialized in orthopaedics, that sell crutches.  In China, you can often find stores in and outside hospitals selling medical accessories.