You can buy length of stay passes at the resort when you check in. Unless you have booked a package that includes LOS passes, you cannot purchase them in advance.
To determine what is the best ticket value, you really have to do the math. Depending on what you plan to do, one of the park hopper plus passes may be a better value than the LOS. Do you plan to be in a park every day of your stay? Go to water parks, Disney Quest and/or Pleasure Island frequently? It may be less costly to purchase a park hopper ticket and pay the admission to the water parks or Disney Quest separately.
On the other hand, if you just want your DS's friend to have the same options as your DS, the LOS is a hassle free way to go.
Based on an eight night stay, my advice is to compare the features and price of the 6 or 7 day hopper plus pass with the LOS option. The links posted by Buzz2001 should help you do this. If you belong to DC or
AAA, you will be able to get a small discount on most ticket types.
Enjoy your trip!