Online or irl? Scan sell them for just over a quid here or Maplin sell them for a lot more (suprise) http://www.maplin.co.uk/3.5mm-stereo-extension-lead-11425 I might even have a spare of the ones from scan lying around somewhere...
I recently found my old PSP inline remote, snipped off the logic connector and am using that. It clips nicely on to my mousemat.
I would avoid the maplins ones the quality is pretty poor as in the cable where it connects to the 3.5 socket become iffy very quickly.
Just found it - it's only 1.5m not 3m like the one I linked to earlier. It's never been used actually - it's still in the little bag from scan! I use one for my SP2500s too - the lead they provide is too short! If you're still interested then drop me a PM GK