The one that I think would be hard would be Canada to US. All I can think of there is Coffee Crisp......Aero??...Kit Kat?? I don't know. There must be a site somewhere that should tell you what other countries don't have, but I do have a couple of ideas of my fave snacks that are local.
I'm having this problem while I've been trying to pre-plan! I've got a little list of things that I can get here and I know I can't get in the States. I'm hoping I get a country that's a little farther away so it'll be easier, but I'm willing to bet there are enough Americans signing up that all us non-us people might get paired up with the States. If that's the case for me I'm really hoping for unique local items, because I only live about 15 minutes from the border anyway, Stateside shopping is a normal thing for me.
Oh well - I'm excited for anything though! Care/snack packages are great no matter what's inside them.

(Also I was told that Aero's were available in the States, am I wrong? Also YOU GUYS DON'T HAVE KIT KATS?)