In terms of ebay I found this seller to be about the best when it comes to C7 double nut (DFU) screws. Bought around a year ago for conversion of an AMAT25LV benchtop mill.
I got my screws down to around 0.015mm backlash using some thin tin foil (approx 0.05mm thickness in total) to pack between the slave nut and spacer.

Or you could spend fortunes on C5 or lower using DFU nut in your searches.