As Bare Feet said enabling path hints helps a lot when trying to find file locations.
However, for this particular file, if I recall correctly, it should be either at:
Best option to do is enabling template path hints.
To enable path hints:
- Login to Magento Admin panel
- Go to System->Configuration.
- The Template Path and Block name will only appear for current website. So change the Current Configuration Scope to ‘Main Website’.
- Go to the Advanced section and click on Developers.
- Expand the Debug tab.
- Set the Template Path Hints to ‘Yes’.
- Click on ‘Save Configuration’ button.
If your website is in production, you can also restrict the hints by IP.