Joe, it would be helpful to have an understanding as to what you are looking for in a bridge. What is the prototype reason for the bridge - valley, river crossing, road crossing, in a town, etc. What prototype materials do you want the bridge constructed of : steel, wood, stone, concrete.
Are you trying to buy something or are you willing to build? Or you can buy some parts and build the rest.
Try looking up a Network Arch Railroad Bridge or Concrete Arch Railroad Bridge. (Found some examples in HO scale but not in O.) I am destined to build one or two of these on my layout to span a planned water feature. If you decide on a girder, I have built the girder sections out of laminated card stock with good results. The type of bridge is somewhat based on the span length and elevation. One would not build a suspension bridge over a short span or where the bottom is close enough for piers.
Also look for the Chenab Arch Bridge - reportedly the worlds highest railroad bridge as a prototype.
Enjoy