I doubt you'll get spare hairlines from anybody still nowadays. So you have to do it yourself. The quick and dirty method is to
+ look for a sheet of transparent plastic ("Plexiglas" or similar),
+ take a needle and a straight edge and cut a fine line in the plastic (there is no need to color this line, the dust will do this for you while you use your slide rule),
+ cut out 2 equal rectangular pieces with the line approximately in their center and perpendicular to one of their edges - the height of the rectangle shall be 1cm more than the height of your slide rule,
+ look for material being as thick as your slide rule,
+ cut out 2 strips of this (5mm high, length = width of the rectangles),
+ take 1 rectangle and adjust it on your slide rule to have the line running *perpendicular* to the edge of your slide rule, then lock it in this position with approximately 5mm standing out on either side,
+ take your 2 strips and glue them on the rectangle next to the top and bottom edge of your slide rule using a liquid adhesive (place a piece of paper between one strip and the edge of your slide rule for proper distance and take care for the glue not touching your slide rule),
+ wait until everything is really fix,
+ place the second rectangle on your assembly, adjust as the first, lock and glue, then wait again.
Done! OK, not too quick - and this will just allow you to use the hair line on either side of the rule, not to transfer anything from one side to the other, but anyway! The glue lines shall take care of the necessary spacing to let the hair line assembly glide over your slide rule. Good luck!