You want a 6" Swiss Pattern #4 Cut Pippin file, which also happens to be the impressioning file of choice, so if you ever get into that then you've already got a file.
Now, these particular files are hard to come by, and generally kind of expensive (for a file), but the best place I can recommend is this:
http://home.earthlink.net/~bradleysmall/locksmithtools
He's got exactly what you want, and $30 is very reasonable (I've seen similars ones for $50-60).
Do get the PVC case; don't get the handle--it doesn't stay on and it inhibits feedback anyway, which is more important.
If you want to go cheaper, then a simple Nicholson small round (rat tail) fine cut file from your local Lowes or Home Depot will work too, although you may have to get a triangular file (small, fine cut) as well and then make the cuts with it and round them out with the round (the Pippin does both jobs, better
)
"I think people should be free to engage in any sexual practices they choose; they should draw the line at goats though."
Elton John
