I don't know if this is a ticket office or what but it says Ticabus in the window.
On Calle 9 just south of Ave 2, there is a place just before the white church. It looks like a ticket office or something.
If you are walking on calle 9 (aka paseo de estudiantes) from Ave 2, it will be on your left side (last store front before the church (iglesia de soledad))
Look at the very left in this photo, you'll see the top of a reddish building, I think that's the place I'm talking about. Again, I'm not 100% sure it's a ticket office. I just remember seeing Ticabus signs in the window.
http://photosynth.net/view.aspx?cid=27d ... 289f9d8a76