where can I buy a garment steamer in Shenzhen?

You can buy it in any Sunning or Gome branch shops. Here is some branch shops of them.
Sunning:
Located at Kangle Building, Xuefu Road, Nanshan District.
Located at Bo. 57-3 Chuangye 2nd Road, Baoan District.
Located at No. 98 – 1 of Meihua Road, Futian area.
Gome
Located at the intersection of Houhai Avenue and Chuangye Road.
Located at No. 1003-2 of Jintian Road, Futian District.
Located at No. 101 of Buji Road.

Thank you, Cindy!