Some airlines, such as AA, allow you to see the current seat map on a flight before you book. I'm not aware of any centralized site that will allow you to search based on current empty seats.

There's of course no way to know that the seats around you won't fill up over time. If you'll really be in pain should you get stuck in coach all flight, the best course of action is to try to at least get an exit row seat, and if you can afford it to fly in business.