You said wanted to search the "source," didn't ya?
To do what you want would require going direct to the database. If you access yours using a tool like phpMyAdmin, the interface makes it fairly easy to search a specific field in a table. Search and replace is a little more complicated, though. For example, when I swapped out
<br /> in my older posts, I used this SQL query statement:
UPDATE wp_posts SET post_content = REPLACE(post_content, '<br>', '<br />');
The MySQL resource page on REPLACE:
EDIT: Forgot to note that it's always a smart move to backup (i.e. export) your database before making any sort of changes to it. Better safe than...you know.