You can try at Obu-City. They have Obu Grape Picking there.
Their opening hours:
days/hours: •SuGWat farmers.inc
Closed September 1st, 2nd, and 3rd for maintenance
[Direct sales] 8:30 AM to 5:00 PM
[Grape picking] 9:30 AM to 3:00 PM (Closes at 4:00 PM)
Access by Public Transport
From Obu Station on the JR Tokaido Main Line and Taketoyo Line, walk approx. 30 min.
You can check their official website where you can also get their phone number.
Here is the link :
•Grape and Blueberry Hills (Budo-to-Blueberry-no-Oka)
Always open during grape picking season