No, there isn't one. (You can just do a sort with the options available to you).
I too think it would be a good idea to have one and it is on our 'what we want to be added' -list, when the site gets a full makeover. At the moment however, building a full featured search function is a little and a lot too much we can handle, but definitely for OFP2 at least.
Good missions are easy to find already though. Just sort by score