You need a 16" Nicholson or Simonds brand, made in the good old USA file if they are the 160gr Snuffers that are 1.5" wide. If they are the smaller Snuffers then you might be able to get by with a 12-14" file. Fastenal, a nationwide industrial distributor has them. Chances are you have a store local or go online to fastenal.com.
Once you get your file go to the 3 blade tutorial that Charlie Lamb has on here. Be sure and take care of your file and it will last a long time. Clean with a solvent like aerosol contact cleaner, acetone, or alcohol first, then chalk it up to keep filings from clogging the teeth. When finished scrub file with file card or fine wire brush, then spray light coat of WD40 to keep from rusting. When you are ready to use the file again start this process all over.
Once they come off the file they should shave. You can then go to a hard arkansas or simply strop them on a cereal box or leather glued down to a wooden block. BE CAREFUL, they are SCARY sharp at this point !