I generally pick up several files per weekend at the flea market. All Simmonds or old (USA) Nicholson. I usually don't pay more than 1-3 dollars a piece. I was getting them to make knives but lately I've been stumbling on to some real gems. I'm thinking about sending some of them out to be chemically sharpened and adding them to my toolbox. I must have 30 or more. Most of the time I anneal them and grind the teeth off them before forging. It's better to start with "bought" steel. Less work involved. (Time is indeed money)
I really have to get back into my shop and fire up the forge. It's been more or less idle for the last year.
ETA: Boy, I'm really out of it this week! To answer your question, I use a big old Nicholson flat bastard. It cuts good & fast. Leaves a pretty smooth surface. I'm sure it is an older file as most of mine are. It's well over a foot long and between 1 1/2- 2 inches wide. Real thick also, a real meaty file.