Where is Tobi's teleportation space location?
It's explained in more detail in the manga. Anime viewers wouldn't know exactly what Tobi's power is.

That location is an alternate dimension, unique to him.


   Actually, since Tobi has the same eye as Kakashi, their dimension is shared. Tobi can freely insert and extract objects to/from his dimension. Kakashi can apparently insert things from a distance, and extract them directly from his eye.

 Kakshi's counter attack happened because Tobi used his Kamui to teleport a part of him (that was going to get hit in the 'real' world), to the alternate dimension. Kakshi was waiting in that alternate dimension and hit him from there (in the stomach, causing him to spew blood). Kakshi later returned to the real world with his own Kamui.