Express passes-buying them in the park questions

[Jenny]

i'm all set to go w/ my tickets & plan on going to ioa & us next tues., 1/22. i decided not to buy the express passes in advance & will play it by ear that day as to whether or not we need them. do they usually run out by a certain time or is there a certain time up until they sell them? also, does the price go down after a certain time? i think that the park is open from 9-6 that day & was just wondering when we should make the decision to buy or not to buy. the price is $25.99 & we would need 2. - thanks - Jen