40 hole rear was
very common on British bikes (matched to a 32-hole front) until they adopted the "Continental" mode of 36-36. If you want to use something other than the EA-3 rim (I assume you now have) and go to alloy, look for some 650A rims, they are the same diameter. 650B are just a bit smaller, and would probably fit, but you won't find them cheap.
Sheldon has all the info in a compact form;
http://www.sheldonbrown.com/tire-sizing.html