The Parelli website.
But I warn you, DVDs are absolutely NO replacement for real-life training. I'm talking about you, not the horse, YOU are the one who needs in-person training so that you can train your horse. DVDs cause more problems then they solve. They cause communication, and very close-minded thinking. A DVD (especially the parelli ones) will only show you one way of accomplishing something. Every horse is different and a certain method doesn't work, there are many more to try. parelli likes to tell you that there is only one way. There are many, so don't get sucked up in thinking there is one.
Parelli preaches a lot of things that can cause some horse to become unsafe. So be careful, and take everything with a grain of salt.
EDIT: I'm always hearing horrible things about Parelli. But I've very good things about Clinton Anderson and Monty Roberts.