The only way to get APs is from Disney. I bought my voucher at a
Disney store - you may want to check your local one. Some seem to carry them, some don't - it's hard to tell after the buy-out. Stores that do have them may just be selling their stock and not getting refilled.
Because of this, there are no discounts anywhere on APs - and the only reason to pre-purchase one is simply to get the expense out of the way. If you order by mail you will recieve a voucher that needs to be turned in at any ticket booth anyway, so again, unless you want to get them paid for ahead of time for some reason it doesn't even save you any time at the counters (just the 30 seconds or so to authorize your card).
Good luck! Being an AP holder is great - takes a lot of the burden off of "we HAVE to get our money's worth" each and every day of your trip. It also is a great excuse to plan more trips, hehe.
N.E.D.
[edit] PS - You may also want to check out the Disney Dining Experience - details over at the restaurant boards. For $50 you get a card good for a year that gets you 20% off of the total bill (drinks included) at virtually all sit-downs and even some counter service (all of the CS at AK and the value resorts, since they don't have disney-owned full-service eateries). If you do the math, they pay for themselves as long as you spend at least $250 on food. The only caveat is that you must have an AP to get one (or be a FL resident), so if you do decide to do it you
will have to buy at least one AP in advance.