Any garden machinery shop should be able to advise - check yellow pages - however...
It's a very simple DIY task - clean blades - buy a sharpening oilstone - dress the sloping edges with the stone - lightly (very lightly) remove any burr from the interior, mating-edges - drop of oil - tighten pivot if necessary.
http://www.youtube.com/watch?v=GgD2O6yj7yM
You do not necessarily need a powered wheel like the guy in the video - hand shrpening just take a little longer.
Good luck...
Peccavi
April 2010