Just as the title says, you should be able to use IE's Find option (or just any Find option, hey) in the 'main' text window of DS. You can use it for browser popups etc, so I don't think this will be especially tough to implement.
Also just wondering, is it possible to add a client/var that holds the current text string the main text window currently shows? It will be even more awesome if it wasn't read-only and you could change it as you wish, actually affecting the text window (having the abillity to remove text from it, clear it, etc as well as read it).
Though I'm afraid this particular suggestion isn't really possible. :\ Maybe for BYOND 4.0?