On weekends, when Oi Keibajo, or Tokyo City Keiba, are not holding horse races, they also hold flea markets. The large-scale markets are held in the car park of the facilities and the recommended time of visit is from morning to early afternoon. The markets to not specialize in any particular thing, meaning that you are able to find almost anything. Japanese antique shopping or second-hand kimono shopping is probably one of the best market activities, with the kimonos selling for only a fraction of what they usually would in stores. There is also a food van/food stall area for you to enjoy lunch after a hard morning of shopping.
The flea market typically starts at 8:30 AM and ends at 2:30 PM. You can expect around 300 - 600 vendors, depending on the day. Entrance is free and only cancelled in case of heavy rain. It's only a 3-minute walk from Ooi Keibajo-mae Station on the Tokyo Monorail, and 10-minute walk from Tachiaigawa Station on the Keikyu Line.