Isetan's supermarket in 1 Mont'Kiara is the best place in Kuala Lumpur to buy Japanese food products: fresh sashimi-grade fish, imported Japanese vegetables, sake and shochu, instant ramen, Japanese condiments, and confectionery. The import range is comprehensive in a way that reflects Mont Kiara's Japanese expat population, and the fresh fish section in particular is a level above what you'll find at Cold Storage or Giant elsewhere in the city.
The supermarket also carries Korean products (kimchi, gochujang, instant noodles) and a better-than-average selection of European and Australian items. Prices are higher than local supermarkets but reasonable for the quality. The sake and beer section is KL's most comprehensive for Japanese brands.