A jalapeno is a jalapeno. However, I've been told, and I've found it to be true so far, that jalapeno's with blunter/rounder ends are milder than those with pointier ends.
If you want something a bit milder, try a poblano. I believe that's the go-to chile for rellenos. I've had some that taste more like green bell peppers. In fact, some even look like bell peppers, but they're not quite as fleshy.
If you're looking to reduce the heat further, get rid of the ribs in the chiles. Some worry about the heat in the seeds, and that's true to some extent, but not so much as the ribs. If you slice one open through the ribs, look for the yellowish streak; that's the capsaicin, the heat. Hottest jalapenos I've tasted had bright orange streaks in them.