I was given the same Thomas bag (bag shaped like Thomas) by my friend who bought it from the airport.
It was in Terminal 2 at a toy shop (can't remember the name) but it was actually next door to a Thomas the Tank engine shop which ironically, didn't have the same bag in store. The shop is located on the second floor of the terminal.
I know you are based in Causeway Bay but I haven't seen this bag anywhere else in HK. Hope this helps.
hapimamee,
in case you haven't brought the bag yet, I saw the one mentioned by Klam (sharp like Thomas) in Tung Chung Shopping Centre today. It's in that Thomas shop and it cost $550.