Straight from the official Killer Instinct Twitter page 
Wondering if itâs a physical copy or just a code that is being sold?
Its already being sold, they just didnât tell anyone it wasnât available until the holidays until after. And âHolidaysâ 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 theyâll make a physical copy for PC.
and i doubt that they will make it playable offline 