Oof! Tough search, to be sure.
However, you might communicate with these folks...
http://www.modelmemories.com/
They may be able to help and/or direct.
Some time ago K&S Metals (Special Shapes Div.) made some etched brass beams in a truss pattern, but I don't believe they continue that product line any longer. I just went to their website and the only 'remnants' of truss beam listings are all marked 'disc.'....not a good sign.
Good luck! If you're successful in finding a source, though, be sure to let us...and the Source List gatekeeper...know.
KD