Actually, you will be searching FOR that term. In the Developers Tool Kit, the lower left block (Look-up in all files) is where you would enter search_in_description. In the next drop-down, select All Files - Catalog/Admin. Select the Case Sensitive box, then click on the search button.
You should have 11 or so instances of the term. Knowing which area of the cart your term is in will help you to weed out the non-applicable files. For instance, you are looking for something that places the term in a file versus one that defines the term.
Then (in this case) you'd look for something that is creating the field in a form. (This term is hidden in a form) This should take you to a tpl_ file.
This search results in 6 tpl_ possibilities but some can be ruled out. For instance, this search is not in a sidebox, so those two files are ruled out.
Through the process of elimination, you'll get to the right spot.
Don't expect to find button_search.gif anywhere in those files because the image is defined as the variable BUTTON_IMAGE_SEARCH in a completely different file.
All this COULD be slightly different from what I've given you here as your template is way different than standard. And, part of the reason it is so different is that they "hard-coded" many items. Sounds like what you're wanting to do, huh?
The whole point in database-driven, css-styled PHP is to avoid hard coding. Once button_search.gif is defined as BUTTON_IMAGE_SEARCH, we can place the variable in thousands of places. When it's time to change the image, we just overwrite the image in the proper images folder. Rather than having to go find where we hard-coded that image, all the images will change automatically.
Hang in there and AVOID shortcuts. Shortcuts are roads to disaster in programming and Zen Cart.
<soapbox>IMHO Template Monster's shortcuts around cart design too often turn into road blocks.</soapbox>