The stamps are at Innoventions, just past guest services. As you enter the park, go to the left of Spaceship Earth. We went the other day for our first before leaving on our own vacation in Illinois, and they had a line set up for the stamps (they also opened early at 9am instead of 11am, but told us it is not guaranteed to open early every day). We got our passports at the store across from Spaceship Earth before we got in line. Just have your AP discount card and ID ready to make it quicker for everyone.