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I recently came across this company that sells whipping twine and they are fairly local to me. So I popped down and purchased some to play with. It turned out to be great stuff, great quality and not at all pricey. They actually sell most of their stuff on eBay and have more than just the port and starboard colours shown below.
Click on Image to see full selection of Whipping Twine:
I can personally recommend that Tarred Line Below. It is a nice hard lay and holds its knot and shape well. Sometimes a good project does look better with tarred twine!!!