If you use Google Adsense then try their search box. I have successfully been using the script for quite some time.
If you don’t use Google Adsense then it is worth joining just for their search box.
Well, for a simple search on the page, you can write your own script for searching, or you can find jQuery plugin for that.
If you need more complex search over all articles/catigories/etc. you can try Google’s custom website search or write server side script . However I don’t sure about your website’s structure, so I’m only guessing.
I highly recommend Zoom Search. You pay Wrensoft for it, but it’s easy to use. To get an idea of how to get it working, see my article: http://iphonedevlog.wordpress.com/2013/09/02/easily-add-search-functions-to-your-app/
2nd vote for zoomsearch. easy to customise too and you can also have it search through linked pages. I have it on my site
Thanks all, I will check out those options!