I think lots of the on-line ticket places have them - I'm pretty sure www.ticketmania.com does (I think I bought some from them). I've used them many times and have always been pleased (and no shipping charges!).
You can also buy them at Kennedy Space Center's web site, but I'm sure you can do better elsewhere.
I think it took us 15-20 minutes to get there. It's not hard, but the directions and signage were not always fully clear once we got on the highways closer to the Space Center. But there are signs and if you miss a turn you will end up at a guardhouse and you can just ask for directions.
For central Florida being a tourist center the signage on all of the roads are lacking.
We get lost almost every time we go, and I'm not sure that we've taken the same way twice from the airport to WDW.
Remember if you want to go east, make sure you're driving north, wanna go west better drive south. If thats not confusing enough they'll give you signs to follow up to the point where you can go either right or left, but they NEVER will have a sign pointing the way you want to go.
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