I saw some at The Lanes in central. Not sure it was the East or the West lanes. But if you walk from The Landmark, the first lane has a costume store on the left hand side. The second lane has a costume stall on the right.
Saw a Santa-style bodysuit with Santa hat in Mothercare. Popi (I think this is the store name) also sell Santa-style suit (either one piece bodysuit or top with trousers and hat) in their store at Discovery Park, Tsuen Wan.
I passed by Popi today in Central. Their Santa Clause costumes are real good. But of course more pricely than those you pick up from the "Stone Street". Price around HKD 168.