Straight from the official Killer Instinct Twitter page 
https://twitter.com/KillerInstinct/status/794917199894970368
Wondering if it’s a physical copy or just a code that is being sold?
Its already being sold, they just didn’t tell anyone it wasn’t available until the holidays until after. And “Holidays” is the same vague time frame they gave a month ago.
I know some of my friends are hyped! 
Where is it being sold?
likely in the windows store or the ingame store of KI. I doubt that they’ll make a physical copy for PC.
and i doubt that they will make it playable offline 