So one of those black covers at the front of the roof rack (the ones on either side of the fairing) just blew off while you were driving? Both of those pieces of trim on my roof rack are missing one of the two clips which holds each of those down. I'm surprised mine haven't flown off yet.
I have been searching for these pieces as well since I broke the clips off and I haven't come across anything yet. I also contacted Nissan and was told that I would have to buy the entire roof rack since they don't sell the parts separately.
Here's my recommendation: Go to
http://www.car-part.com and enter in your information. In the 'Select Part' drop down box (also, choose sort by distance), choose 'Roof' and then select 'w/o sunroof' on the next screen. There should be a decent amount of auto recyclers near you with R50's with a roof intact. Most likely there will be at least a few that have the SE roof rack with those black pieces intact. You can always just call them up and ask if they have them before driving there.