Ray,  I'm not sure which German supplier you refer to, but Herzog
will supply in pieces about 18 inches by 8 inches (the size varies
a little).  I had some only a few days ago.

Contact Jutta Kiffe <jutta.kiffe@lederfachhandel.de> who should be
able to help.  They sell three sizes (size zero is the size I mentioned
above, the others are smaller), and three thicknesses: extra-thin, thin
and medium.  I think extra-thin is the best for bird boxes.

Regards,
John Farmer, UK
http://www.musicanic.com/