There are a variety of options for marimba bar cord. Some prefer a softer cotton cord for less bar noise, some prefer a more durable nylon cord, and some use cord with a nylon or even wire core covered with a softer cotton sheath. The marimba manufacturers will sell bar cord at a high mark-up or you can purchase in bulk. One common version is parachute cord, available from military surplus stores. I recommend the kind that has seven internal strands, covered with a braided sheath. Also see www.cheaperthandirt.com — they have 100-, 300-, or 1000-foot spools in black or colors.