I bought the huge Wilson US Open balls in Mongkok in March. There is one sports shop(always put a huge yellow Head racquet in the window you can see) in 花園街. In the opposite of 洗衣街花園 but I don't have their contact no. Also 玫瑰体育用品 may still have one left as i saw before. There is is just nearby 鵝頸橋 where is near Times Square in Causeway Bay.
玫瑰体育用品 Rose Sports Goods & Sports Clothing
湾仔 人和悦大厦 Yan Wo Yuet Bldg, Wan Chai