How do you get that green hat on your avatar? Is it possible anymore to get? Do you have to buy the green hat?

Fish auctioned it some years ago, only some people got it. You can't get it anymore.

I heard that he made an event about it not auction.

it was an auction. the event you're talking about is probably the one about the first 3d items, where the green hat 3d item was first seen.