Time Out says
Around 700 stalls line the streets of Setagaya during its biannual flea market, held on January 15-16 and December 15-16 each year. The Setagaya Boroichi has been going for over 400 years now and remains one of the highlights in the neighbourhood's calendar, drawing tens of thousands of visitors each time. You'll find most of the stalls along Boroichi Dori, a street which centres on the Setagaya Daikan Yashiki, the old, thatched-roof local magistrate’s residence. Expect to see the longest lines at stalls selling the market’s popular Daikan Mochi rice cakes, where it’s not uncommon to wait over an hour to get your hands on one (tip: timing is everything). We’d recommend hopping off the train at Setagaya Station on the Setagaya Line, walking along Boroichi Dori, and then leaving from Kamimachi Station. Note that tickets are bought either when you get on the train or as you enter the platform, so you’ll have a smoother trip if you get the fare ready beforehand (¥140 for adults, ¥70 for children).