Heavy cream in Japan is 生クリーム, sold in 200 ml cartons in the dairy case. Check the label so you don't pick up the vegetable-fat whip by mistake.
生クリーム
nama kurīmu · real dairy cream
ホイップ
hoippu · often vegetable fat, not cream
In the shop
Dairy case, next to milk and butter. Before Christmas it also appears near the baking goods.
Which shops
Every supermarket. Gyomu Super and Costco sell 1 L cartons.
Watch out
Look for 種類別 クリーム (type: cream) and 乳脂肪 (milk fat) 35–47% on the side. Cartons labelled ホイップ or 植物性脂肪 are vegetable fat: fine for topping a cake, but they split in sauces.
Lazy Recipe sees that you're in Japan and turns any dish into 3 steps with what Japanese supermarkets sell. Your shopping list shows each item's Japanese label and aisle.