Tabletop Simulator released on June 5, 2015 and can be purchased at these official distributors:
Steam, Humble Store, Greenman Gaming, Bundle Stars Store, and Amazon.
(sites not listed here are NOT authorized retailers)
You can also use one of the widgets below to purchase the game directly. When buying a 4-pack, please remember that if you already own the game, you will only receive 3 copies from Humble, or with Steam you won’t be able to purchase a 4-pack because you already have a copy. So what you need to do is purchase a 4-pack as a gift to someone else so they can distribute it. This applies to both Steam and Humble.