How do get special avatar for boards?

If you mean things like the Arbiter and Moderator gemstones/icons you have to be part of those respective teams in order to get them.

You can get the Boards Enhancement Kit here; Boards Enhancement Kit (BEK)

Just post there what you want for your avatar (must be boards appropriate) and they'll give it to you. Keep in mind that it will only be seen by those who also use BEK, but a lot of the regulars do.