Where to find tennis ball basket & balls?

Try the tennis shops around Victoria Park near Tin Hau.

Marathon and/or Gigasports in Pacific Place.

I'm not sure that pacific place will have the item, it may be a little to specialist for either Marathon or Gigasports to stock but you should check. I do think the tennis shops at Victoria park will have them or be able to refer you to a shop that does stock them, in particular there is a shop called the pro shop which is a specialist tennis shop (at least it was there when I was a kid and tennis player). Its on electric road, come out the MTR (Victoria Park exit, A2) and its one of the first shops on the right just next to a fruit shop. If you don't find it there are quite a few sports shops on that road that cater very specifically to tennis on account of the Victoria Park tennis courts being just across the road.