I agree 100% with smrobs.
Parelli stuff is very expensive, and yes, it is DVD sets (plus they recommend that you get all of their brand of halters, whips, etc.), and the cheapest you'd probably get away with finding them would be on Ebay. I was able to buy some Stacy Westfall DVDs pretty cheap off of Ebay and I really like those. I've also been to a Clinton Anderson seminar and I have TONS of books and magazines from John Lyons, who happens to be my favorite.
Look around, keep an open mind, and figure out what works for YOU.