Where can I purchase

Hi,
1) You can go to any JR office or station.
2) No, and you really can't online anyway. Cheapest seat is an unreserved seat - ¥13,600.
3) Easiest way is to go to the Hommachi Subway Stn and take the Midosuji Line to Shin-Osaka. As for Shobuya, it is a large district, so it depends where you are going.
4) Trains start running around 6 AM or so. They stop near midnight. See Hyperdia.

Good luck.

Hello Toraneko!
Thank you for your reply.
We will br staying in Lions Mansion Dougenzaka in Shibuya. Does this help?
What's the difference between reserved seat and unreserved seat besides having the seats reserved? I mean if we choose the unreserved seats, we still get to sit right but it's more of a first come, first serve basis, am I right?
Thank you.

Hi,
Well, you can easily look these things up on Google maps. You would be very close to Shibuya Stn.
As for unreserved seating, yes you are right. Some train cars are for reserved seating only, and others are unreserved. There is no guarantee of getting a seat with unreserved seating, but unless you're traveling during a peak season, it's quite unlikely that you could not find one.

Thank you very much! ^^,