Most dealers will have lots of these kicking around in their junk boxes - they may give it to you for nothing or charge you just a nominal amount. But insulating or gaffer tape will work just as well.
Gusboll:What the hell is an optical end cap?If you look at the back of your cyrus cd8 you will see one in your optical out.Small black plug to keep dirt out.
The little blanking plug in the optical digital input/output sockets on equipment. Some more recent equipment just has a little shutter that's pushed aside by the plug being inserted.