That yoga is flexibility training is a misconception.
Yoga works synergistically on flexibility, balance, strength, endurance. Vinyasa (flowing from pose to pose) can even have a cardio effect based on how the routine is put together.
The type of yoga poses you practise can emphasize flexibility versus the other aspects.
Athletic stretching is also a good way to develop flexibility, but yoga is what has worked for me, so I recommend it.
Any yoga practice DVD that emphasizes flexibility will help you more than a yoga DVD that has more holistic-training objectives.
I personally like the Yoga Zone titles (they are old but still great DVDs, available quite affordably at amazon) that have a flexibility focus.
Stretching for Flexibility
Flexibility and Stress Release
I also recommend Rodney Yee's AM yoga for your week. It has five 20-ish minute session so it can be tagged on after a strength or cardio workout. Each session has an emphasis - forward bends, backbends, twists, etc. So you get to work on each plane in rotation. The cueing is outstanding. It isnt specifically for flexibility but the emphasis on other aspects is more muted.
AM Yoga For Your Week
You could also try Yin yoga/restorative yoga with props, but I find it very boring. You get into and hold a yoga pose for a long time in yin styles of yoga. Un-propped, I like it, but it isnt suitable at all for those with limited flexibility. The props ensure you dont over-extend and actually help you slowly develop range of motion, but they make it sooo cumbersome.