How about an auto parts store? Looks exactly like some stuff we used WAYYY back when I had my 1978 Trans-Am to put the 12 speakers in.
Take a picture along and they might be able to match something.
http://www.mnpctech.com/UChannel.html - WAY cheaper
Mike
Online will probably be cheaper, but since you cant buy online...
OH you might want to check places that install car stereos also - they might have the stuff around too.