Hmm i dont think there is a script like that you need to make it custom.

I dont think it would really be that hard for the coder

1: Just add ask box, similar to search box
2: Use ajax on the site so loads on the same page without reloading
3: The google auto complete is called Google Suggest

I think ask a coder like Mr Happy here on KWWHunction to get you a quotation for this.