Most large supermarkets will have either an Asian section or International section where you can buy specialty Japanese items.
Read about the Essential Japanese Pantry Ingredients here
Asian Supermarkets are also great because they stock a large range of authentic ingredients that can be hard to find.
Some of my favourite Easy to Cook Japanese Food at home is
Tonkatsu (Deep Fried Pork Cutlet)
Gyudon (Beef Bowl)
Karaage (Japanese Fried Chicken)
Nikujyaga (Meat & Potatoes)
Easy To Follow Steps To Create Over 108 Top Japanese Recipes!