MegaLowRider.com is a U.S. source of chain guards. They have several variations of the one chocula posted.
Azor produces work bikes for the European market. You may be able to get a full chain case from one of their dealers.
Biria and Electra both sell bikes in the U.S. with chain guards/cases, and you may be able to get a replacement one through their dealers.
In all cases they are going to be a challenge to mount on a frame not designed for them.
The easiest to mount chain guard I've seen is also one of the most elegant, made by Sogreni.


--A