When you say it broke off, did it fall in? I'm assuming that you're talking about the plastic piece that's two concentric circles right?
If you push hard/deep enough, the center piece actually comes out. So it may be that you have a piece somewhere underneath the plastic.
Obviously you could ask the dealer, but I bet they charge $5 for ONE. I've never seen these type of fittings elsewhere, but Home Depot/Lowes might have something like this. Or you could go the auto route and try Pep Boys.
No I mean break off! I am NOT talking about the ($5) small nylon rivets that you have to push in in order to remove... Behind the inners, the nylon rivets hold on to a plastic peace. There is one on each side of the upper side of the widscreen. They are actually the same color as the bike; my 2002 Busa is Blue & Black the plastic peaces are also Blue and Black ). I pushed the windscreen and on side broke off!
One has to look on the top-inside of where the windscreen slide in to see it.