Hi Justin. I am not sure where to get a genuine nikon o-ring, but if you have a caliper or fine measuring implement, try measuring the o-ring from the nikon camera that failed and ordering a replacement off McMasterCarr. McMasterCarr is fabulous for this application. They sell many types of O-Rings for various applications.
I had to order a replacement O-Ring for the top sprayer arm for my GE dishwasher. Ge only sells the O-ring as part of the top sprayer kit, which retails for $100. I searched McMasterCarr and was able to order an FDA listed food-safe O-Ring that fit within the temperature requirements of my dishwasher. Pretty fantastic.
For o-rings, it is best to buy a few around the size you need. So one or two sizes up in cross sectional width and maybe one or two larger in outer diameter. Just in case you measured wrong, you don't have to wait another week to get the right part! O-Rings are usually pretty cheap, so it's worth the redundancy.