I would look on Craigslist if I were you for a companion. To introduce a new duck to a flock, keep it in quarantine for about a week and monitor it away from your flock. If it's looking healthy, then put it in the coop at night, so the other duck doesn't go after it. Plus, it's less stressful all around for the newbie at dark.
You could also go to your local Tractor Supply Co. leave a “Wanted” sign or ask if you could have the contact for their duck partner. All TSC get their ducks from locals so they may have older ducks willing to sell!