All kayak, general marine, or diving shops should carry several variations of the same basic design. Most are well under $10. A Fox 40 is probably as good as any.
The whistle should be carried on your person while kayaking. It won't do you any good in a drybag in your front hatch...thats where the harmonica belongs... to insure you plenty of privacy at the campsite. Most people attach the whistle to their PFD with a lanyard or clip made for the purpose.
There are several whislte codes that are used to communicate between people. The problem with these codes in paddling, especially whitewater where canyons and river valley walls generate echoes, is that the whistle signals can be confusing. You may let out one blast, but echoes cause another person to hear two or three. Similarly, you can blow three blasts only to have the other paddler miss the first one and think you blew two.
For that reason, do not use a whistle for general communication. It should be reserved for emergency signalling only.