You can integrate the DuckDuckGo search box to search your site using this wizard. There's a great writeup on how to change the style of that box over here (archived).
If you'd like to add a handy search box that simply searches DuckDuckGo through your page, you can add this code to your site:
<form method="get" action="https://duckduckgo.com/"> <input size="40" name="q"> <input type="submit" value="Search"> </form>
If you have other needs or integration ideas, please let us know!