Refinery, after upgrading the Milk farm, at the basement floor of the New Ark Lab, buy it from them.
Then to the Milk farm insert Milk into the refinery.
Also, you're wrong, the refinery is on the 3rd floor of the upper new ark mall, for 2000 credits.
Said as much(except the price), on the wiki: https://lustdollplus.miraheze.org/wiki/Milk_Farm
And I just checked ingame.