When you say that Lightspeed no longer has replacement parts, do you mean there isn't a part listed on the website, or you've talked to them about out-of-warranty repair work on a Legacy headset? If it's just the cable, the repair should be fairly simple.
Lightspeed also offers a tradeup program for the 15XL on up (20XL, 3G, etc), up to 400 toward a new headset.