This isn’t default Magento. I’m assuming you’re using a pre-built theme which would have the blocks listed in the layout xml file. Depending on what page the left column is used on, depends on what xml file the block declaration would be in.
For example, if the left column is viewable from the category page, you would look for:
(Change package and theme to your theme’s package and theme name)
If the left column is viewable from CMS pages you would look for:
These xml files won’t hold the specific html that you are looking for but would instead most likely have a block calling to a phtml file somewhere in:
Unfortunately, I can’t help much more than this without knowing what theme you’re using. If it’s free I don’t mind installing it locally and searching for the specific html (assuming it’s not modified theme code....)