It’s not in one location, unfortunately. Not being a developer or designer, you might find this a hard system to get a handle on..
anyway, the layout XML files use a line reference="left" or reference="right" for items in the right or left column.
If you turn on template path hints, you will have more success traveling the trail to find the right files:
navigate to: Admin > System > Configuration
change “Current Configuration Scope” to the actual store (drop down menu, top left)
When you then navigate to Admin > System > Configuration > Develope(tab on the right)r > Debug(tab in main area) the template hints option will be available.