You'll probably get more answers on the "Bicycle Mechanics" forum. Lots of knowledge about obscure parts there.

My guess is that you should shop around for a cheap, double-ended gear cable. Look in the non-bike-shop world- I've seen them at department stores. Cut off the end you're not going to use and you're set.

Better gear cables generally go with better shifters, which all use the barrel end cable that you have.