Yamaha DTXpress/DTXplorer/DTXtreme

I feel like a really dummy. I have the standard DTXpress1 kit, plus a 
bar pad, and I've only just twigged that I can virtually double the 
number of pads on my kit by splitting each stereo input. What I can't 
seem to find is a conversion cable plug (two mono jacks to one stereo 
plug). I have tried asking at the electronic-part stores here in 
Adelaide, Australia, but the best they can come up with is an adaptor 
that converts two stereo jacks to one stereo plug. But that doesn't 
do the trick - both pads will produce the same sound (i.e. the pad 
tuning, not the rim tuning). I'm told I'll have to make up my own 
patch cables, which I'd rather not have to do. Can anyone tell me 
where I can buy conversion cable plugs online?