Check out this link:
  http://search.ebay.com/search/search...&fsop=1&fsoo=1  
You can find about a bazillion choices for bells and you shouldn't have to spend $25 on a bell just because it's for "dog training". 
There are plenty of "service" bells out there for much less that will work just fine.  As for how to train them to use it, there is alot of info/articles online on this subject.  Also, any good dog training book utilizing positive reinforcement methods should cover this topic.