Search & Rescue Dogs

Can anyone recommend a whistle to help me train my collie/labrador?

I use the Acme Fox 40 Micro whistle from ebay

I use an acme 211 and half whistle, the 211 and half is the code relevant to the pitch of the whistle, of which there are a few. I would also purchase a couple. I did have 4, (one for working, one in car, one at home and a spare) I have 1 left.
My advice also would be not to use string to put the whistle on, use something that will give easily, I had a bad experience with a branch and the string, when out working one of my dogs.

Thats a good point, I keep mine on a safety lanyard - the type of thing office workers keep their ID tags on, it had an easy break section which stop it strangling you.