You can also make them by yourself, I'm using glass or A mirror.
It's also flat and I already had it, so it was an easy pick.
The downpoint of glass and A mirror is that it's quite heavy and fragile, so it's not the best pick.
You could also use A thick peace of Lexan, plexiglass, acrylic or maybe even carbonfiber, it's lighter and it's also stronger than te other two.
If you would like to have A real board, companies like Hudy and Integy make some good quality setup boards.
And if you really want something different, McMoney makes custom pitboards!
