Where can I buy one of these.

Turner Richards sell them so do most pet shops.
Is there any reason you want a silent whistle? As a trainer I always advise people that using a silent whistle is all very well until the day the dog ignores it and you then have to ask yourself is it working? I advise people keen to whistle train their dogs to use one they can also hear, then there is never that element of doubt over whether the dog did hear you or not.

My thoughts exactly.  The only purpose I can see in a silent whistle is for "covert" operations! :D :D :D

I ask because someone on CD advised that they used one to stop their dog from barking. When Gabi is barking she is so wound up that she does not hear any commands. It seems that the silent whistle was used to get a seconds quiet so that the dog hears your commands.

I haven't heard of this use for one, just goes to show you learn something new in dog care every day! Thanks for letting me know, I shall bear that in mind for the future, you never know when it might be useful.