Let's see...

Hobby Shop K has some
http://www.hobbyshopk.com/dual.html

There's also
http://www.kcat.zaq.ne.jp/collection/dual.htm
(don't know the name of this place, all-Japanese, 
but I found the home page - 
http://www.kcat.zaq.ne.jp/collection/ - from 
cels.org)

Animaxis also had some for sale recently...
http://www.animaxis.com/en/index.asp

Tell me if you find any more places!

Alan Zabaro