I've Lost One of my Earphone Sleeve

As usual, the very last thing i did before head out from my room to go to work was strapping my iPod to my belt, plugged an earphone into my ear, and start playing music at random. This morning, i sense something wasn't entirely right with my right ear. It significantly quieter than the left one. I ignored the fact by attributing to the, most probably, increase of my right ear's wax amount. Yeah, gross, i know. So, i went to work anyway.

This noon, however, when i had a time to examine my Sennheiser MX160 (pictured), i noticed the cause of why my right ear hear less through the earphone. I've lost one of the earphone sleeve (circled in red in the picture). A couple of minutes Googling, i don't find any helpful info of wether they sell the spare earphone sleeve or not. Is anybody had any idea on where i should buy the earphone sleeve? Or is there any such thing as spare earphone sleeve?