It looks likes you have two options:

There is an [<kbd>ALT</kbd> Code](https://en.wikipedia.org/wiki/Alt_code) you can try to type.  To try it:

- Hold down the <kbd>ALT</kbd> key on your keyboard
- Using the number pad (it must be the number pad), type 2, then 1 (21)
- Release <kbd>ALT</kbd> and it should type a §

On Mac, it looks like if you hold <kbd>ALT</kbd> and type 6, it will type the § sign.   

The other way is to use plugins that replace the § with something else.  Looking around, it seems [this plugin](https://dev.bukkit.org/projects/commandbook) may work.


Sources: [Minecraft Wiki](http://minecraft.gamepedia.com/Formatting_codes#Typing), [Talk:Formatting codes](http://minecraft.gamepedia.com/Talk:Formatting_codes#How_do_you_get_.C2.A7_sign.3F)