While people do keep one duck all the time without issue, breeders generally don't recommend it.
I would say you should get another duck to keep your's company. The breed really isn't too significant, but if you want to possibly breed them yourself you should certainly get a male pekin.
It is a bit difficult to find a place that will sell just one duckling. However, eFowl.com will allow you to buy one duckling, but be mindful that they may include warming chicks (extra birds), in the package to keep the ducklings safe during shipment.
Ducks are very social creatures so it's very cruel to keep just one. Call aPoultry farm and see if they can get you another duck or a goose as a companion. I called one and they are going to find a goose or a duck for me because one of my ducks is missing and left the other one all alone. : (
We just lost our female Pekin and now just have the one male Pekin and two hens...should we get another duck for our male or will he be satisfied with the hens as companions?