The functionality is already there; it's just not obvious. To perform a "find and replace", you first do a "find" (cmd-f), then type in the replacement over the selection. You can then do a "do again" (cmd-j) to find and replace the next occurance, or (ctrl-j) to find and replace all remaining occurances. – Tim Olson
Tip: You do not have to perform a "find" for the "replace" to work. Just select a word you want to have replaced, overtype it with your new word, and hit (ctrl-j). Very handy for renaming variables. – Vanessa Freudenberg