The best way is to pick one up at the little twin gift shops on the walkway from Future World to World Showcase, so that you can have it stamped in each country as you go around.
I haven't done one in years, but I think these days you get them stamped at the Kidcot stations in each country.
They are a lot of fun for an inexpensive souvenier (I think they are about $6), and make for a terrific keepsake.