Try the electronic stores at the southwest corner of ZhongHua / ZhongXiao Rd., i.e. just north or Ximending / west of Peimen (Northgate).
Seen some converters in the window of one of the shops there.
Alternatively try the underground shopping mall in the southwest corner of the GuangHua market area (not below the bridge!), close to the junction of XinSheng and BaDe. There are some electronic shops in the basement (B1 / B2)