I'm working on a LEGO project with my son and we want to include a bridge. However, I cannot find individual arch pieces that would work for a bridge.
This sounds like a fun project! There are lots of options available for arches. It might be helpful to know a bit more about what you are looking for. If you are trying to build an arch bridge such as this:
The 18838 arch or similar might be a good fit. These can be purchased on Bricklink or directly from LEGO fairly inexpensively.
If you don't have an easy way to get that piece or a regular arch piece doesn't meet your needs, you can get pretty good results emulating arches of any shape using inverted slopes (or regular slopes if you get clever). Here's an example brick-built arch by Tom Alphin:
You can find arched bricks produced by LEGO on Bricklink. You can also find available colors for each item as well as purchase if you need extra.