These hints are mainly given for German people because they are not familiar to USA sources.  

amazon.com. 

Totalfitnessdvds.com. You must enter "10off"  in the coupon code then you get 10 % reduction

Collagevideo.com.

You can also order directly from Kelly Coffey or Cathe Friedrich. If Kelly releases a new DVD she holds a presale. Check out from time to time.
coffeyfit.com .  It is also possible to download DVDs. However, you get only the workouts without the great premixes, which make the training more varied.

Cathe Friedrich offers  daily deals  Cathe.com.  Here it is also possible to download, unfortunately without premixes.

Additionally Cathe offers "Cathe live" for 9,97 $ per month. Every Thursday Cathe gives live lessons in her gym at 9.15 . These lessons are recorded. You can take part live. All recorded lessons are archived and you can retrieve all archived lessons whenever you want. "Cathe live" has been available since May 2014. You can imagine how many workouts there are by now (boxing, interval training, spinning, total body training....)

There is another offer.  "Cathe on Demand"  for  19,97 $/ per month. It is possible to retrieve not only the archived lessons but also all workouts from her released DVDs (more than 300) and there is even a workout blender.