You can not get a discount AP, not even advance purchase, and the only place to buy them is through Disney. Current costs for APs run $336 for ages 9 and under & $395 for ages 10 and up, Premium (includes water parks, pleasure island, disney quest) run $438 for ages 9 & under and $515 for ages 10 & up. You can buy a voucher(s) in advance (to help off set the cost) and then just activate them when you get to the parks--they don't start "running" until you activate them. I believe, but am not sure, allearsnet.com and even mousesavers.com has a list of the other perks and a calculator if it is a good deal for you.
FYI: Costs of other tickets are:
Water Park Single Day Adult $34 & child $28
Disney Quest Single Day Adult $34 & child $28
Pleasure Island Single Day $20.95
Disney's Wide World of Sports® complex Adult $9.81 & Child $7.24
Magic Your Way varies on what you choose to have