The conbini in the Hilton is open 24 hours, but it's not an actual 7/11. This is an important distinction for those relying on the "7/11 ATMs accept foreign ATM cards" feature - the only ATM in the Hilton is the one in the conbini, and it does NOT accept foreign cards.
We found that American food was a bit more expensive in the grocery store in Ikspiari, but did well by bringing our own Clif bars and Tang (mmm, just like the 80's, but with less grit!) to save on breakfast costs. For other snack type items choose the grocery in Ikspiari or the 7/11 as others have already mentioned.
Oh, and there are fun Disney themed snacks in the hotel stores too!
Drop another coin in slot and I will tell you more.
*sigh* unfortunately, I'm lactose intolerant so it's been decades since I've bought milk in Japan. I know that there is "Milk" (Regular) and "Low-fat". But if I remember correctly, the "regular" has less fat than in US.