I have the same double butted spoke on my bike and the correct nipple size is 14 gauge. Here's a note from my bike store:
> The 14/15 gauge measurement on the spoke actually refers to the fact that they are a butted spoke, 14g at the ends and a narrower 15g in the middle (to make them lighter but still strong)... Yes, 14G is the size for the spoke nipple.
The next thing to determine is the correct length of the nipple you need since they come in varied lengths. Note that it is easy to make a mistake on the length, unless you have samples to compare sizes. I had to return a purchase 14G/14mm since the right size is 14G/16mm.
Good luck.