You need to search for PARALLEL MACHINE FILES. Good luck with that!
I had the same problem about 12 years ago, easier to get bull feathers! I eventually bought some from Federal file co in the USA for my bench machine (files, not feathers), they are a similar size to Swiss files. (I have an unused set of these PM if interested). I could not find any new or used files for my large filer so I bought a set of Swedish files (no particular reason for buying Swedish except the shop had a set in stock that looked as if it would do) 12", round, flat, square, tri, half round, etc. I cut off the non parallel parts and ground the ends to fit my machine, 60 degree point at bottom, 1" square tang at top to fit chuck, cuts on down stroke. Medium or coarse files are better, you will die of old age if you use fine files on these machines. If you haven't used a filing machine before, hold downs are essential if you value your fingertips.
Harold Hall designed a small filing machine some years ago, he made some suggestions on modifying standard files, should be some info on his website. Daveb