Check the Walthers catalog. They sell a lot of kits that could probably help.
Faller is a big maker of brick as well as stone bridges in plastic kit form.
Hi kase: How high is that bridge going to be? If you are going to make it very high, you will need some grade work to get to the height of the bridge. European models would be a good place to start if you want to stay with brick work.
I would make it out of wood, probably about 2" wide. I would just build a sturdy wood frame and then glue a brick veneer to the sides. I built a huge concrete viaduct this way once, and used masonite for the veneer.