The file I use is a triangular jeweler's file. They're also referred to as needle files. I personally call them tripes, that's a local term, so others may not know this label. For engraving, I use the sharp edges to cut lines.
Anyhoo, try looking at
Micro-Mark for file sets.
On the homepage, select "Product Search", then "Files and Sanders", then "Files". There are several sets to choose from, but I'd look at the first and fourth sets, as well as the fifth set from the bottom. They look like they contain a triangular file.
You can also try a local hobby shop if you have one, a craft store, or a hardware store. Just ask for small jeweler's or needle files, and be sure to mention that you're specifically looking for a three-sided or triangular file.
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