Download spocalc: http://www.sheldonbrown.com/rinard/spocalc.htm. Theres a rim database there. If you can find your rim, just look for others with same erd. If not, there are instructions for measuring erd if you scroll down the download page I linked you to.

Even easier is to choose the rim you want, and then buy spokes to fit it. Plus, you get to take measurements and use spocalc, which is fun and educational.