If you're building your own twine sensors or need to replace the cable that came with the twine, you might find them at a TJ-Maxx store. I was in the store looking at the iPhone accessories on a rack near the checkouts. I found a twin-pack of 3.5mm audio cables with a male TRRS plug at each end. (They had several)
The cables are 5 feet long, and one is black and the other is white. The brand is iSound (www.isound.net) and the package of 2 was $7.99. At the iSound web site they are $9.99. Look for model "ISOUND-1609".