The best gun dog whistles are found in stores across the world. Your gun dog can be used for hunting and also for a companion. You may want to first begin dog whistle training months before you take the dog hunting for the first time. This way you can easily train the dog first before going out on the prey.
Most of the time, the dog will respond at far distances with just about any sort of high pitching whistle. For training purposes you may need to look into a premium whistle. I always would look at doing training with the same whistle as I use for hunting.