Arlington: How does one go about finding a good headhunter? I don't know of anyone who's used one in my function (marketing), so I can't get a recommendation, and I'm at a loss as to where to start.
Have you tried Google? Seriously. I did a quick search for "marketing careers recruiters" and turned up oodles of leads. The other thing to keep in mind is that not all fields use recruiters heavily to fill jobs. If no one you work with has used one, take that as a sign to rely on other tactics -- primarily networking.
Washington: I work in a mid-size nonprofit in a small, yet important department. I am about one-third the total manpower. My current boss has only been here about a month. My previous boss has stayed on board, half-time, as a senior adviser. I am actually in salary negotiations with a potential new employer, and I would like to give the typical two to three weeks' notice. But should I offer my current employer more time to find a replacement before I move on? What's standard?
Two weeks is standard, as you know. Three weeks would be very generous of you. I wouldn't advise stretching it out longer than that. While you just want to be helpful, most organizations are loath to have short-timers hanging around the office. It's bad for morale. You could, however, offer to be available for any questions from your replacement until he or she gets settled in.