I just had my saw (Ryoba) sharpened. A gentleman I met while working on a job in New England sent it to a friend of his in Vermont who did a beautiful job. Tuned it up a bit -it had "acquired" a bit of a bow in the blade. It's now nice and straight and much sharper than when it left me.
Japan Woodworker sells the various sized feather files used to sharpen the saws but I would submit to you that this is a little trickier than sharpening a Western saw. I mentioned in an earlier post that HIDA might have a sharpening service available. They did at one time. I should also clarify that all this talk of sharpening refers to the nicer handmade saws, not the disposable-blade saws. Don't get me wrong, I like my Silky disposable-blade saws very much too!!