Kappabashi is the largest kitchenware shopping street, near Asakusa and Ueno. You can find lots of specialty stores selling kitchenwares you've never seen but one of the famous store there is the one selling plastic food samples.
4 shops introduced here:
In the store called Ganso Shokuhin Sample-ya, they even offer workshops to make food samples!
Information seems to be only in Japanese but it seems to be around 3,000 yen (depending on what you want to create) taking about an hour.