Yeah it's here! Really it's right here.
I am relatively new to the hobby. I wonder how/where can you usually get promos (except buying them on amazon/ebay)?
As already stated, Promos are dependent on what and where they are doing the promotion. Some are 'sweetners' to get higher donations during a Kickstarter campaign, some or promos included in a magazine to encourage you to subscribe, some are released at a convention to encourage you to go to the con and visit thier booth, a few are web exclusives to drive you to thier own website.
So, short answer, 'depends'.
As mentioned, many end up in aftermarket or BGG marketplace.