I use the C7/C14 adapter that has round sides on both sides of the adapter. I think you need the C7/C14 "polarized" version that has round on one side and square on the other:
http://www.takefiveaudio.com/mall/shopexd.asp?id=361
if that one doesn't match your requirements then I'm out of ideas...
http://www.takefiveaudio.com/mall/shopexd.asp?id=361
if that one doesn't match your requirements then I'm out of ideas...