There are many department stores in Shinjuku 新宿 area of Tokyo.
Why don’t you go to ISETAN 伊勢丹 to look around the B1 floor?
You can buy so many kinds of foods and sweets for to go.

Business hours 10:30am – 8:00pm

It looks delicious!

The food that is displayed is priced by “100g (3.5oz)” and is sold by weight.
When you order this type of food, please order how much you want by weight or you can also order by price.

There are food section and sweets section on the B1 floor.

It looks delicious!

Honestly, the prices are not cheap but they are tasty.

Onigiri おにぎり Rice ball

Tempura

Website
ISETAN 伊勢丹 in Shinjuku 新宿
http://isetan.mistore.jp/store/shinjuku/index.html

Google map
The other floors are good for shopping such as cosmetics, jewels, clothes and so on.