Exchange money at Post Office

Hello

I have to exchange some money, but I can't figure out what post offices do have some money exchange service... I remember that I could change money at Meguro Post office, which was quite big, and is reasonably close to my house, but are there others in the same area?
(Also, is there a website where I can find that I tried the JP Post website, but couldnt find an answer)

Also, when are the service hours for post offices?
As I'm a student, I have classes from 8AM to 6PM, so I'd like to know if there is some place where I can exchange money either after 7PM or on a Sunday...

Thank you

From the post page:

Small post offices are open Monday to Friday from 9:00 to 17:00 and are closed on weekends and national holidays. Larger offices are opened on weekdays until 19:00 and may also open on Saturdays and Sundays. And a handful of major offices are open even longer. For example, the Shinjuku office provides most services until 21:00.

And some official information:
http://www.post.japanpost.jp/english/openhours/index.html

You can look up individual office locations and hours here in Japanese:
http://map.japanpost.jp/pc/