Proper nut files are extraordinarily expensive, and the cheaper ones not really good at all for that job. If it's just to get rid of the 'jerking' of the string over the nut when tuning, a light sprinkle of graphite powder does a fine job. Some folk get the graphite from quality pencil lead, but it's easy enough to obtain. Amazon propose various versions, as a little flaçon of 50ml for 9€86, or a 1 litre tub for 10€10, and everything in between. Try that first, if unsmooth tuning is the issue..?