Yes. There's a grocery store in the basement of Ikspiari (or the 1st floor, I forget if the subway station is considered the 1st floor or bus stop is 1st floor). People leaving the park sometimes go there to buy post-Disney snacks because it closes at 10:30 or 11. There's also one just past the entrance fountain on the right but I've never gone there because its operating hours are shorter.
If all else fails, you should be able to find those things in the convenience stores at Maihama station or some of the hotels.