Option 1: Drop $150 on the steel Nitto Jaguar stem. It comes in similar angle to the Cinelli 2A and has 25.4 clamp.
Option 2: Get a pair of Cinelli 67-39 Pista bars with a 26.4 clamp. My opinion is that these are superior to any bar Nitto makes.
Option 3: Shim. Get an old aluminum can and cut out a few narrow strips the width of the stem. Or you can spend $10 on the Nitto shim, but that will only get you .6. They're designed to take you from 25.4 to 26.0.