I ordered it from Amazon (Black Plastic Drag Chain Cable Carrier 10 x 15mm for CNC Router Mill). It was a tight fit, you might want to go to with 10mmx20mm.
The Drag Chain would get caught on one of the exposed bearings on top, so I used a scrap piece of acrylic I had laying around to support the DC. I got darn lucky that the acrylic was just long enough:)
BTW, it took FOREVER for the Drag Chain to arrive, via slow boat from China.