The easy way to measure scale rope is to closely wind ten turns (for thick rope) and 20 or 30 turns (for thin rope) around a dowel. You then measure the total width and divide by the number of turns. This is much more accurate than trying to measure with a caliper or micrometer which tend to flatten the line.
Scale rope is not thread. There's a difference. Thread consists of a single strand. Scale rope, like full sized rope, is made of three strands. It looks different. The most convenient source is Model Expo. They carry a full line of scale rope from several different manufacturers. A cheaper source, if you can find it, is to go to a very good sewing supply center and look at the button thread. Often, such thread is (1) made out of silk or polyester which is much better than the usual cotton, (2) is pre-waxed which helps a lot in rigging, (3) is three-stranded and may be available in both left-hand and right-hand wind, (4) far cheaper.
There are both multi and single strand threads available. One does have to look and be careful to only get the multistrand stuff, but it is out there at good fabric/sewing/craft shops. Monofilament means single strand- not what you want. Many cotton threads are 3-strand.
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