agree with Moose
Mel Kreiger's Essence of Flycasting videos are excellent for beginner through intermediate and on to advanced casters (Essence of Flycasting II). These two single hand videos together are probably the best available to take a caster from beginner through intermediate casting skill development. Bill Gammel also has a great tape called Teach Yourself to Flycast (well, I think that is the correct title, as I've lent it to a friend who--you guessed it--is teaching himself how to flycast)--I learned a lot about how to teach single handed casting by watching and reflecting on Bill's tape. Joan Wulff's Dynamics of Flycasting is also very good, as are the Doug Swisher casting videos by 3M (I think they are called Basic Fly Casting and Advanced Fly Casting).
And of course for the intermediate -to-advanced caster who really wants to add efficiency, distance and power to their casting, the absolute best treatment of the subject is George Roberts's Saltwater Fly Casting: 10 steps to distance and power.
Any of these would be good for your customers, MJC...
Loop Canada Pro Staff
FFF certified casting instructor