I've got one from HS Strut called Mouth Calling 101, and one from Primos called Mastering the Mouth Call... both were available as combo deals including the CD Rom (which is what both of those are) as well as some mouth calls packaged with them...

They're both good, but if these are 101-level courses, I almost wish there was a 100-level course... I'm pretty dense when it comes to using diaphram calls; but then I can't roll my tongue when trying to say a Spanish word or anything either...