I get most of my files from flea markets, as well as most of my punches, chisels, drifts, etc. I use them as is or modify for what I need them for, and generally pay a quarter to a dollar each.
I have a separate set of really nice modern files for more precision work, but those get used more in woodworking than smithing for me personally.
Making your own tooling can be very rewarding and modifying existing tooling can be a good way to get started.