Flea markets are the answer to the sometimes high prices in department and grocery stores. They happen all around the Nagoya area at various times in the year and even many on a monthly basis. The ones I have encountered thus far are set outside like stalls at a fair. Many of the flea markets have been held at temples.
Kosho-ji temple in Nagoya, near Irinaka, is a prime example. You can find everything from clothes to cheap apples as is the case with flea markets anywhere. The thing that makes them good is the price of produce. Instead of 400 yen for six apples at the store, you can pay 700 yen for 15. The price of tomatoes is a must to look at being 6 for 700-800 yen whereas they go for 120 yen each at the store.
A key mark of a flea market in Japan is that there will always be stands that offer food prepared on the spot with deep fryers, skillets or grills. Flea markets are more like fairs. There is also, always lots of fish. among that, you will find assortments of clothes, especially socks. If you like food that is definitely Japanese then flea markets are a good place to find takoyaki and dango.