thomann also sell one.
goes for about a fiver, but you'd have to factor the postage in to the equation.
i have one of the thomann ones and i find to be a very funtional piece of kit.
BTW, i must say that i have tried the mp3 case thing as well, but didn't work too well for me. I found that i couldn't do it up well enough to hold it - as I'm a pretty active monster when it comes down to it, and i didn't like running the risk of stomping on the transmitter!