To use a file you would have to grind off all the teeth down to where there is no hint of them remaining, otherwise every tooth would act as a stress riser and make your spring want to break. A carbon steel file can theoretically be spring tempered, but finding out exactly what alloy it is for the heat treat profile, that's another matter. Unless you are somewhere UPS does not go, it's going to be easier/faster/more effective to buy spring steel from somewhere like McMaster. If that's too expensive, how about using an old car or trailer spring from a junkyard?