Originally Posted by tropple

Plans at the bottom of the page:
http://www.pavekmuseum.org/theremin/index.html

A whole site about a guy's Theremin:
http://mrr3000gt.mystarband.net/MRT/index.htm

This site sells kits:
http://paia.com/theremax.htm

My editor is really into these things. I think they sound cool, but I haven't enough time or interest to pursue that, too.