I don't believe that there's any way that you can purchase them online. The Disney World official site lists them as being available only at the gate.
However, you do have a couple of options to save you time in line:
1.) If you're staying at a Disney resort, purchase them at check in. They can encode them onto your room key, thus saving you the paper ticket. It'll all be on one card, and it's all taken care of in one line!
2.) If you are NOT staying at a Disney Resort, call your hotel now and find out if they sell 1-day, 1 park admission to the Disney parks. You may be able to purchase them at check-in at an off-property hotel as well.
3.) Make a stop at Downtown Disney. I believe they can sell you a ticket at Guest Services. I went to inquire about days remaining on a hopper pass, and there was only one other person there. I would have been done in no time flat, but their computer system locked up!
4.) Call 407-WDISNEY now, and ask them. I don't know whether or not they'll sell you one day admissions on the phone, but it's worth a shot. I believe the thinking behind this is that they want to sell you a multi-day ticket. They can't try and upsell you if they can't talk to you, which might explain why you can't purchase online.
5.) As someone else suggested, just get a good jump start on the day, and arrive early.
I hope it all works out for you. Have a fantastic trip!!!!
