Buying them ahead of time is good. Our first trip to WDW, I had no idea how long it took to get all the stamps from the countries. I bought it on our last full day at Epcot and later in the day. I did not get all of them stamped. I went back the next night after MK and tried to run to get more stamped. Boy did I not realize how huge EPCOT was. BY the time I got to Mexico I was so tired from running. I had to get to Japan, which was where we left off. I didn't make it. I stopped by Guest relations on my way back out. I explained that this was our first trip, and that my daughter really wanted all the stamps. She was 3 then. They said they would take it around to get it stamped and mail it to us. Sure enough about 2 weeks later I get the passport, with really nice writings from everyone who signed it.
It was wonderful.
Melanie