I've always ordered online so I can't really tell about getting one in the park but cost is enough for me to purchase it in advance first. I also don't like the idea of having to take touring time to go do something that can be done before we leave home.
As for timing, I was really shocked at how fast we received it. I don't recall exactly but I think it took 4 days max.... maybe even 2 after we ordered it.
Just to make sure we're talking of the same thing though, when you order the photopass before you go, they don't actually send you anything. You get an online password that you use to order your pics once you get back. You always have to get the actual photopass card from the park and they're free... all photographers carry them. You can also have more than one so if the family splits up, both sets can get pics taken. When you get home, you plug in the numbers on the back of all your cards into the one account and you can see all the pics from all cards at once and place your order for the cd.
HTH