Unlike some very popular boards (eg RPi) I don't think there are many (if any? - others can shout at me if I'm wrong ;-) 3rd party commercial 'hats'/expansion boards. Some contributors have built their own expansions for their own needs (browse through the Projects forum to see what others are doing), but as for commercial boards specifically targeted at the Onion devices, I believe that the Onion store will be your likely source.
Having said that, the point of the devices (IMHO) are to spark your own ingenuity and get you to explore and build your own projects and if that means taking it to the board level, then that is what you should do, as have others. I see the commercial expansions from Onion as tools for prototyping and in some cases, all that is necessary to realise your own project. But the flexibility is there for you to do with them as you choose - or not even use them at all and instead design your own.