Hi, can I buy a prepaid sim card in Hangzhou Airport? Thanks

Well, as I know, no shops at the airport sell the SIM Card. So you are suggested to buy it in downtown area.
There are many branches of China Mobile and China Unicom. You can buy a card at these places.
China Mobile:
No.165, Zhaohui Road, Xiacheng District.
No.207, Tiancheng Road, Jianggan District.
China Unicom:
No.501 to 509, Yanan Road, Shangcheng District.
No.131, Banshan Road, Gongshu District.