There aren't many Z scale bridges, period. For trusses, we have a couple of really nice brass kits from TrainCat (if they're ever released), and the make-believe brass kits from Micron Art. There's also an old wood pony truss from RSLaserKits.
If you want a cheap plastic steel truss, you can start with the fantasy-land Atlas one-piece N scale truss (below), chop it apart, and reassemble it, along with some Evergreen strip stock, into a fairly acceptable Z scale through or deck truss.
