Suitable arched bricks for building 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: